Original differential revision submitted by @linimon at https://reviews.freebsd.org/D16946.
pxeboot(8): remove antiquated information
While I was there:
- Fix typos
- Fix an excessive argument "indent" reported by mandoc -Tlint
- Replace a dead link with the one suggested by https://www.uefi.org/uefi
Submitted by: linimon (in part)