docs: Improve inline docs for permissions (deno run --help) (#18757)

Hey there! I took a crack at improving these embedded docs [as requested
here](https://github.com/denoland/deno/issues/18685). These should
accurately reflect the functionality of the permission-related flags for
`deno run`.

### Highlights
* Adds human-readable argument string in the format [prescribed in the
docs](https://docs.rs/clap/latest/clap/struct.Arg.html#method.value_name)
* Keeps text description terse, but includes a relevant copy/pasteable
docs link
* Includes example argument usage/formatting

cli/args/flags.rs

@@ -1882,6 +1882,90 @@ fn compile_args_without_check_args(app: Command) -> Command {
     .arg(ca_file_arg())
 }
 
+static ALLOW_READ_HELP: &str = concat!(
+  "Allow file system read access. Optionally specify allowed paths.\n",
+  "Docs: https://deno.land/manual@v",
+  env!("CARGO_PKG_VERSION"),
+  "/basics/permissions\n",
+  "Examples:\n",
+  "  --allow-read\n",
+  "  --allow-read=\"/etc,/var/log.txt\""
+);
+
+static ALLOW_WRITE_HELP: &str = concat!(
+  "Allow file system write access. Optionally specify allowed paths.\n",
+  "Docs: https://deno.land/manual@v",
+  env!("CARGO_PKG_VERSION"),
+  "/basics/permissions\n",
+  "Examples:\n",
+  "  --allow-write\n",
+  "  --allow-write=\"/etc,/var/log.txt\""
+);
+
+static ALLOW_NET_HELP: &str = concat!(
+  "Allow network access. Optionally specify allowed IP addresses and host names, with ports as necessary.\n",
+  "Docs: https://deno.land/manual@v",
+  env!("CARGO_PKG_VERSION"),
+  "/basics/permissions\n",
+  "Examples:\n",
+  "  --allow-net\n",
+  "  --allow-net=\"localhost:8080,deno.land\""
+);
+
+static ALLOW_ENV_HELP: &str = concat!(
+  "Allow access to system environment information. Optionally specify accessible environment variables.\n",
+  "Docs: https://deno.land/manual@v",
+  env!("CARGO_PKG_VERSION"),
+  "/basics/permissions\n",
+  "Examples:\n",
+  "  --allow-env\n",
+  "  --allow-env=\"PORT,HOME,PATH\""
+);
+
+static ALLOW_SYS_HELP: &str = concat!(
+  "Allow access to OS information. Optionally allow specific APIs by function name.\n",
+  "Docs: https://deno.land/manual@v",
+  env!("CARGO_PKG_VERSION"),
+  "/basics/permissions\n",
+  "Examples:\n",
+  "  --allow-sys\n",
+  "  --allow-sys=\"systemMemoryInfo,osRelease\""
+);
+
+static ALLOW_RUN_HELP: &str = concat!(
+  "Allow running subprocesses. Optionally specify allowed runnable program names.\n",
+  "Docs: https://deno.land/manual@v",
+  env!("CARGO_PKG_VERSION"),
+  "/basics/permissions\n",
+  "Examples:\n",
+  "  --allow-run\n",
+  "  --allow-run=\"whoami,ps\""
+);
+
+static ALLOW_FFI_HELP: &str = concat!(
+  "(Unstable) Allow loading dynamic libraries. Optionally specify allowed directories or files.\n",
+  "Docs: https://deno.land/manual@v",
+  env!("CARGO_PKG_VERSION"),
+  "/basics/permissions\n",
+  "Examples:\n",
+  "  --allow-ffi\n",
+  "  --allow-ffi=\"./libfoo.so\""
+);
+
+static ALLOW_HRTIME_HELP: &str = concat!(
+  "Allow high-resolution time measurement. Note: this can enable timing attacks and fingerprinting.\n",
+  "Docs: https://deno.land/manual@v",
+  env!("CARGO_PKG_VERSION"),
+  "/basics/permissions\n"
+);
+
+static ALLOW_ALL_HELP: &str = concat!(
+  "Allow all permissions. Learn more about permissions in Deno:\n",
+  "https://deno.land/manual@v",
+  env!("CARGO_PKG_VERSION"),
+  "/basics/permissions\n"
+);
+
 fn permission_args(app: Command) -> Command {
   app
     .arg(
@@ -1890,7 +1974,8 @@ fn permission_args(app: Command) -> Command {
         .num_args(0..)
         .use_value_delimiter(true)
         .require_equals(true)
-        .help("Allow file system read access")
+        .value_name("PATH")
+        .help(ALLOW_READ_HELP)
         .value_parser(value_parser!(PathBuf))
         .value_hint(ValueHint::AnyPath),
     )
@@ -1900,7 +1985,8 @@ fn permission_args(app: Command) -> Command {
         .num_args(0..)
         .use_value_delimiter(true)
         .require_equals(true)
-        .help("Allow file system write access")
+        .value_name("PATH")
+        .help(ALLOW_WRITE_HELP)
         .value_parser(value_parser!(PathBuf))
         .value_hint(ValueHint::AnyPath),
     )
@@ -1910,7 +1996,8 @@ fn permission_args(app: Command) -> Command {
         .num_args(0..)
         .use_value_delimiter(true)
         .require_equals(true)
-        .help("Allow network access")
+        .value_name("IP_OR_HOSTNAME")
+        .help(ALLOW_NET_HELP)
         .value_parser(flags_allow_net::validator),
     )
     .arg(unsafely_ignore_certificate_errors_arg())
@@ -1920,7 +2007,8 @@ fn permission_args(app: Command) -> Command {
         .num_args(0..)
         .use_value_delimiter(true)
         .require_equals(true)
-        .help("Allow environment access")
+        .value_name("VARIABLE_NAME")
+        .help(ALLOW_ENV_HELP)
         .value_parser(|key: &str| {
           if key.is_empty() || key.contains(&['=', '\0'] as &[char]) {
             return Err(format!("invalid key \"{key}\""));
@@ -1939,7 +2027,8 @@ fn permission_args(app: Command) -> Command {
         .num_args(0..)
         .use_value_delimiter(true)
         .require_equals(true)
-        .help("Allow access to system info")
+        .value_name("API_NAME")
+        .help(ALLOW_SYS_HELP)
         .value_parser(|key: &str| parse_sys_kind(key).map(ToString::to_string)),
     )
     .arg(
@@ -1948,7 +2037,8 @@ fn permission_args(app: Command) -> Command {
         .num_args(0..)
         .use_value_delimiter(true)
         .require_equals(true)
-        .help("Allow running subprocesses"),
+        .value_name("PROGRAM_NAME")
+        .help(ALLOW_RUN_HELP),
     )
     .arg(
       Arg::new("allow-ffi")
@@ -1956,7 +2046,8 @@ fn permission_args(app: Command) -> Command {
         .num_args(0..)
         .use_value_delimiter(true)
         .require_equals(true)
-        .help("Allow loading dynamic libraries")
+        .value_name("PATH")
+        .help(ALLOW_FFI_HELP)
         .value_parser(value_parser!(PathBuf))
         .value_hint(ValueHint::AnyPath),
     )
@@ -1964,14 +2055,14 @@ fn permission_args(app: Command) -> Command {
       Arg::new("allow-hrtime")
         .long("allow-hrtime")
         .action(ArgAction::SetTrue)
-        .help("Allow high resolution time measurement"),
+        .help(ALLOW_HRTIME_HELP),
     )
     .arg(
       Arg::new("allow-all")
         .short('A')
         .long("allow-all")
         .action(ArgAction::SetTrue)
-        .help("Allow all permissions"),
+        .help(ALLOW_ALL_HELP),
     )
     .arg(
       Arg::new("prompt")