guids.txt: Fix up some guid descriptions
guids.txt: Fix up some guid descriptions

Some of the descriptions of what GUIDs are used for were pretty
ambiguous; this hopefully makes them more clear.

Signed-off-by: Peter Jones <2e0a021e603479ff81838c00c3b770daca4e0214@redhat.com>

rhboot/efivar,rhboot/efivar,rhinstaller/efivar,rhinstaller/efivar