_content/doc/comment: document form of directive comments
_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 <5ad239cb8a44f659eaaee0aa1ea5b94947abe557@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website