Documentation: git-fetch/pull updates.

Documentation: git-fetch/pull updates.

We do not accept multiple <refspecs> on one Pull:/Push: line
right now (we could lift this tentative workaround for the
broken refnames), but we have always accepted multiple such
lines, so use that form in the examples and discussion.

Also explicitly mention that Octopus is made only with an
explicit command line request and never from Pull: lines.

Add a couple of cross references.

Signed-off-by: Junio C Hamano <junkio@cox.net>