loader manuals: Clarify console and boot_serial

console=comconsole is for the loader and boot_serial is for the kernel.

I think we should centralize documentation on the loader settings in one
manual, such a loader.8. The current proliferation is not obvious where
to find the documentation. To the user, these are all set in loader.conf
or at the loader prompt.

MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D53695

(cherry picked from commit 09ffaa4e07d749c83cc1db13829505f21b025f29)