FreeBSD Handbook: FreeBSD Boot Process: directions for users of GNU GRUB

Use uppercase for GRUB.

Prepare a link to the frequently asked question about GRUB. In the pull
request: pauamma suggested not linking from the Handbook, to FAQs,
until all FAQ translations are at least as current as the corresponding
Handbook translation.

Add a note, directing users of GRUB to GNU-provided documentation.

Reviewed by: pauamma, bcr, gjb (mentor)
Approved by: gjb
Pull request: https://github.com/freebsd/freebsd-doc/pull/87
Differential revision: https://reviews.freebsd.org/D36681