push: mention "push.default=tracking" in the documentation

Change the documentation for push.tracking=* to re-include a mention
of what "tracking" does.

The "tracking" option was renamed to "upstream" back in
53c40311 ("push.default: Rename 'tracking' to 'upstream'", 2011-02-16),
this section was then subsequently rewritten in 87a70e4c ("config doc:
rewrite push.default section", 2013-06-19) to remove any mention of
"tracking".

Maybe we should just warn or die nowadays if this option is in the
config, but I had some old config of mine use this option, I'd
forgotten that it was a synonym, and nothing in git's documentation
mentioned that.

That's bad, either we shouldn't support it at all, or we should
document what it does. This patch does the latter.
Signed-off-by: NÆvar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -2432,6 +2432,8 @@ push.default::
   pushing to the same repository you would normally pull from
   (i.e. central workflow).
 
+* `tracking` - This is a deprecated synonym for `upstream`.
+
 * `simple` - in centralized workflow, work like `upstream` with an
   added safety to refuse to push if the upstream branch's name is
   different from the local one.