git fetch doc: add a new section to explain the ins & outs of pruning

git fetch doc: add a new section to explain the ins & outs of pruning

Add a new section to canonically explain how remote reference pruning
works, and how users should be careful about using it in conjunction
with tag refspecs in particular.

A subsequent commit will update the git-remote documentation to refer
to this section, and details the motivation for writing this in the
first place.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>