Go documentation can have header lines, which are single-line paragraphs
with leading and trailing letters and almost no punctuation.
Before this CL, the only allowed punctuation was ' followed by s.

After this CL, parentheses and commas are also allowed,
to pick up a pair of previously unrecognized headings in the
go command documentation:

	Gofmt (reformat) package sources
	Modules, module versions, and more

Change-Id: I6d59c40a1269f01cef62a3fb17b909571c2f2adb
Reviewed-on: https://go-review.googlesource.com/122407
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>