man: document LoaderEntryLastBooted

man: document LoaderEntryLastBooted

The option was introduced with systemd v250 although it went
undocumented. It effectively sits between Default and OneShot, in term
of priority.

It is repeatedly updated as long as loader.conf default is "@saved" and
the OneShot is not set.

v2:
 - squash some typos

v3:
 - special mention to default @saved + loader.conf reference
 - reword update side, to avoid misleading that it's needlessly
   overwritten

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>