_content/doc: document retract directive
_content/doc: document retract directive

For golang/go#33637

Change-Id: Ifd33478f89496a06a51c9f6b9662928093163698
Reviewed-on: https://go-review.googlesource.com/c/website/+/286952
Trust: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>
Reviewed-by: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>

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