release_managers_guide: add BLEAD-POINT/-FINAL

release_managers_guide: add BLEAD-POINT/-FINAL

the Release Manager's Guide talks about three different release types,
RC, BLEAD and MAINT. This is a bit ambiguous for a major release
(e.g. 5.14.0): is it the first MAINT or the last BLEAD?

So, split BLEAD into two:

BLEAD-FINAL - a final release on a blead branch (e.g. 5.14.0)
BLEAD-POINT - a point release on a blead branch (e.g. 5.15.0, 5.15.1)

Then update the whole document to use these tags consistently.