glossary: Update and rephrase the definition of a remote-tracking branch

The definition of a remote-tracking branch in the glossary have been
out-of-date for a while (by e.g. referring to "Pull:" from old-style
$GIT_DIR/remotes files).

Also, the preceding patches have formalized that a remote-tracking branch
must match a configured refspec in order to be usable as an upstream.

This patch rewrites the paragraph on remote-tracking branches accordingly.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/glossary-content.txt

@@ -423,12 +423,13 @@ should not be combined with other pathspec.
 	linkgit:git-push[1].
 
 [[def_remote_tracking_branch]]remote-tracking branch::
-	A regular Git <<def_branch,branch>> that is used to follow changes from
-	another <<def_repository,repository>>. A remote-tracking
-	branch should not contain direct modifications or have local commits
-	made to it. A remote-tracking branch can usually be
-	identified as the right-hand-side <<def_ref,ref>> in a Pull:
-	<<def_refspec,refspec>>.
+	A <<def_ref,ref>> that is used to follow changes from another
+	<<def_repository,repository>>. It typically looks like
+	'refs/remotes/foo/bar' (indicating that it tracks a branch named
+	'bar' in a remote named 'foo'), and matches the right-hand-side of
+	a configured fetch <<def_refspec,refspec>>. A remote-tracking
+	branch should not contain direct modifications or have local
+	commits made to it.
 
 [[def_repository]]repository::
 	A collection of <<def_ref,refs>> together with an