deno doc docs (#5650)

docs/tools/documentation_generator.md

@@ -1,3 +1,33 @@
 ## Documentation Generator
 
-<!-- TODO(lucacasonto): write things -->
+`deno doc` followed by a list of one or more source files will print the JSDoc
+documentation for each of the module's **exported** members. Currently, only
+exports in the style `export <declaration>` and `export ... from ...` are are
+supported.
+
+For example, given a file `add.ts` with the contents:
+
+```ts
+/**
+ * Adds x and y.
+ * @param {number} x
+ * @param {number} y
+ * @returns {number} Sum of x and y
+ */
+export function add(x: number, y: number): number {
+  return x + y;
+}
+```
+
+Running the Deno `doc` command, prints the function's JSDoc comment to `stdout`:
+
+```shell
+deno doc add.ts
+function add(x: number, y: number): number
+  Adds x and y. @param {number} x @param {number} y @returns {number} Sum of x and y
+```
+
+Use the `--json` flag to output the documentation in JSON format. This JSON
+format is consumed by the
+[deno doc website](https://github.com/denoland/doc_website) and used to generate
+module documentation.