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 53c4031 ("push.default: Rename 'tracking' to 'upstream'", 2011-02-16), this section was then subsequently rewritten in 87a70e4 ("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: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>