MFC r245910,r247370,r249371,r252492,r252586:

r252586:

Add descriptions to the FILES list, fix some mdoc complaints, move the
"neither" entry out of the table in BOOTING.

r252492:

Add a new gptboot(8) man page.  Factor out the redundant information
in gpart(8) and boot(8), adding references to gptboot(8) in both.

r249371:

Remove kernel options from the SYNOPSIS. They are already documented in the
geom(4) manual page SYNOPSIS.

r247370:

Fix typo in EFI GPT GUID.

r245910:

Expand description of how gptboot and gptzfsboot choose a partition for
booting.
MFC r252696:

Put the power modes in a list to make them easier to find.
MFC r252713:

Whitespace fixes: remove tabs on blank links, spaces following content,
tabs after spaces.