go/doc: add support for arbitrary notes

Add support for arbitrary notes of the form // MARKER(userid): comment
in the same vein as BUG(userid): A marker must be two or more upper case [A-Z] letters.

R=gri, rsc, bradfitz, jscrockett01
CC=golang-dev
https://codereview.appspot.com/7322061

Committer: Robert Griesemer <gri@golang.org>