rebase docs: clarify --merge and --strategy

rebase docs: clarify --merge and --strategy

Add a paragraph about the swapped sides in a --merge rebase, which was
otherwise only documented in the sources.

Add a paragraph about the effects of the 'ours' strategy to the -s
description.  Also remove the mention of the 'octopus' strategy, which
was copied from the git-merge description but is pointless in a
rebase.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>