To structure larger sections of comments in html output headings
are detected in comments and formated as h3 in the generated html.
A simple heuristic is used to detect headings in comments:
A heading is a non-blank, non-indented line preceded by a blank
line. It is followed by a blank and a non-blank, non-indented line.
A heading must start with an uppercase letter and end with a letter,
digit or a colon.  A heading may not contain punctuation characters.

R=jan.mercl, gri, adg, rsc, r
CC=golang-dev
https://golang.org/cl/5437056