_content/doc/comment: document form of directive comments

Fixes golang/go#43776.

Change-Id: I3e18dbe5dfb0bca96a16203efd4c2a92636cf412
Reviewed-on: https://go-review.googlesource.com/c/website/+/442516
Run-TryBot: Russ Cox <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: rsc

_content/doc/comment.md

@@ -490,6 +490,11 @@ For example:
 	//go:generate stringer -type Op -trimprefix Op
 	type Op uint8
 
+A directive comment is a line matching the regular expression
+`//(line |extern |export |[a-z0-9]+:[a-z0-9])`.
+Tools that define their own directives should use the form
+`//toolname:directive`.
+
 Gofmt removes leading and trailing blank lines in doc comments.
 
 ### Paragraphs {#paragraphs}