Documentation: flesh out “git pull” description

Documentation: flesh out “git pull” description

The current description in the pull man page does not say much more
than that “git pull” is fetch + merge.  Though that is all a person
needs to know in the end, it would be useful to summarize a bit about
what those commands do for new readers.

Most of this description is taken from the “git merge” docs.

Now that we explain how to back out of a failed merge (reset --merge),
we can tone down the warning against that a bit.

Except, as Thomas noticed, there’s a risk with that because people
might read this version of the manpage online and then conclude that
it is safe to try a merge with uncommitted changes, only to find that
their “git reset” doesn't support --merge yet.  Or worse, verify that
their git-reset has --merge by a quick test (1b5b465 is in 1.6.2) but
then find that it does not help with backing out of a merge (e11d7b5
is only in 1.7.0!).  So keep the warning.

With clarifications from Ævar, Thomas, and Junio.

Noticed-by: Geoff Russell <geoffrey.russell@gmail.com>
Cc: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Cc: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>