bhnd(4) update inline documentation comments and man pages.

This includes a number of copyedits for the inline code documentation
comments, updates to the existing bhnd(4), bhndb(4), bcma(4), and siba(4)
man pages, and new man pages for bhnd_chipc(4), bhnd_pmu(4), bhndb_pci(4),
bhnd(9), and bhnd_erom(9).

Approved by: adrian (mentor)
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D13021