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