git remote doc: correct dangerous lies about what prune does

git remote doc: correct dangerous lies about what prune does

The "git remote prune <name>" command uses the same machinery as "git
fetch <name> --prune", and shares all the same caveats, but its
documentation has suggested that it'll just "delete stale
remote-tracking branches under <name>".

This isn't true, and hasn't been true since at least v1.8.5.6 (the
oldest version I could be bothered to test).

E.g. if "refs/tags/*:refs/tags/*" is explicitly set in the refspec of
the remote, it'll delete all local tags <name> doesn't know about.

Instead, briefly give the reader just enough of a hint that this
option might constitute a shotgun aimed at their foot, and point them
to the new PRUNING section in the git-fetch documentation which
explains all the nuances of what this facility does.

See "[BUG] git remote prune removes local tags, depending on fetch
config" (CACi5S_39wNrbfjLfn0xhCY+uewtFN2YmnAcRc86z6pjUTjWPHQ@mail.gmail.com)
by Michael Giuffrida for the initial report.

Reported-by: Michael Giuffrida <michaelpg@chromium.org>
Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>