Expand documentation describing --signoff
Modify various document (man page) files to explain
in more detail what --signoff means.
This was inspired by https://lwn.net/Articles/669976/ where
paulj noted, "adding [the] '-s' argument to [a] git commit
doesn't really mean you have even heard of the DCO...".
Extending git's documentation will make it easier to argue
that developers understood --signoff when they use it.
Signed-off-by: David A. Wheeler <dwheeler@dwheeler.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>