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>