The earlier documentation made vague references to "is set to build
on". Flesh that out with references to the config settings, so folks
can use git-config(1) to get more detail on what @{upstream} means.
For example, @{upstream} does not care about remote.pushdefault or
branch.<name>.pushremote.
Signed-off-by: W. Trevor King <wking@tremily.us> Signed-off-by: Junio C Hamano <gitster@pobox.com>