stand: Document that boot0 uses BIOS

And thus has a limited range of supported baud rates. Also add that
setting BOOT_BOOT0_COMCONSOLE_SPEED=0 will leave it unchanged which
sometimes can give you 115200 if the BIOS initialized things outside of
the normal BIOS baud rates (which many x86 enbedded-targetted boards
do).

Sponsored by: Netflix
Reviewed by: emaste, manu (earlier versions)
Suggestions by: jhb
Differential Revision: https://reviews.freebsd.org/D36300