push doc: correct lies about how push refspecs work

push doc: correct lies about how push refspecs work

There's complex rules governing whether a push is allowed to take
place depending on whether we're pushing to refs/heads/*, refs/tags/*
or refs/not-that/*. See is_branch() in refs.c, and the various
assertions in refs/files-backend.c. (e.g. "trying to write non-commit
object %s to branch '%s'").

This documentation has never been quite correct, but went downhill
after dbfeddb12e ("push: require force for refs under refs/tags/",
2012-11-29) when we started claiming that <dst> couldn't be a tag
object, which is incorrect. After some of the logic in that patch was
changed in 256b9d70a4 ("push: fix "refs/tags/ hierarchy cannot be
updated without --force"", 2013-01-16) the docs weren't updated, and
we've had some version of documentation that confused whether <src>
was a tag or not with whether <dst> would accept either an annotated
tag object or the commit it points to.

This makes the intro somewhat more verbose & complex, perhaps we
should have a shorter description here and split the full complexity
into a dedicated section. Very few users will find themselves needing
to e.g. push blobs or trees to refs/custom-namespace/* (or blobs or
trees at all), and that could be covered separately as an advanced
topic.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>