doc: mention tracking for pull.default

When looking at a configuration file edited long time ago, a user
may find 'pull.default = tracking' and wonder what it means, but
earlier we stopped mentioning this value, even though the code still
support it and more importantly, we have no intention to force old
timers to update their configuration files.

Instead of not mentioning it, add it to the description in a way
that makes it clear that users have no reason to add new uses of it
preferring over 'upstream', by not listing it as a separate item on
the same footing as other values but as a deprecated synonym of the
'upstream' in its description.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -1749,7 +1749,8 @@ push.default::
   +
   This is currently the default, but Git 2.0 will change the default
   to `simple`.
-* `upstream` - push the current branch to its upstream branch.
+* `upstream` - push the current branch to its upstream branch
+  (`tracking` is a deprecated synonym for this).
   With this, `git push` will update the same remote ref as the one which
   is merged by `git pull`, making `push` and `pull` symmetrical.
   See "branch.<name>.merge" for how to configure the upstream branch.