efibootmgr: minor cleanup (spdx, typos, see also)

This page was getting pulled into apropos unix results due to
arguments being on the same line as a name macro in synopsis.
While here, tag spdx, fold a line slightly better, add loader.efi(8)
to see also and fix it's order.

MFC after: 3 days
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1430