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: NMichael Giuffrida <michaelpg@chromium.org>
Signed-off-by: NÆvar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-remote.txt

@@ -172,10 +172,14 @@ With `-n` option, the remote heads are not queried first with
 
 'prune'::
 
-Deletes all stale remote-tracking branches under <name>.
-These stale branches have already been removed from the remote repository
-referenced by <name>, but are still locally available in
-"remotes/<name>".
+Deletes stale references associated with <name>. By default, stale
+remote-tracking branches under <name> are deleted, but depending on
+global configuration and the configuration of the remote we might even
+prune local tags that haven't been pushed there. Equivalent to `git
+fetch --prune <name>`, except that no new references will be fetched.
++
+See the PRUNING section of linkgit:git-fetch[1] for what it'll prune
+depending on various configuration.
 +
 With `--dry-run` option, report what branches will be pruned, but do not
 actually prune them.
@@ -189,7 +193,7 @@ remotes.default is not defined, all remotes which do not have the
 configuration parameter remote.<name>.skipDefaultUpdate set to true will
 be updated.  (See linkgit:git-config[1]).
 +
-With `--prune` option, prune all the remotes that are updated.
+With `--prune` option, run pruning against all the remotes that are updated.
 
 
 DISCUSSION