style: Allow C++ comments

Attempt to craft the more-or-less conesnsus view on C++ BCPL-style
comments. Allowing them seemed to have wide support. Consistent use was
highly desired. What constituted consistent use, however, didn't have
good consensus especially with multi-line comments. Craft something
that's a little vague to allow what appears to be a range in common
practices elsewhere and no good way to drive this do absolute agreement.

We should revisit this point in 6-12 months after we get some experience
with it.

Sponsored by: Netflix
Discussed with: many in the code reivew, see discussion there
Differential Revision: https://reviews.freebsd.org/D35960

(cherry picked from commit 0a59a6b02fccef82644439a5165b7e0b751049c3)