libc/stdbit: add man pages for stdbit functions

This adds man pages for each group of functions in <stdbit.h>.
The man pages have cross references to one-another.
Cross references from external man pages to these will be added
in a later commit.

Reviewed by: pauamma@gundo.com, kib
Approved by: markj (mentor)
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D53659