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>