4d35924e (Merge branch 'rr/triangle', 2013-04-07) introduced support
for triangular workflows, but the push.default values still assume
central workflows.
Rewrite the descriptions of `nothing`, `current`, `upstream` and
`matching` for greater clarity, and explicitly explain how they
should behave in triangular workflows.
Leave `simple` as it is for the moment, as we plan to change its
meaning to accommodate triangular workflows in a later patch.
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>