Documentation: clarify and expand description of --signoff

Documentation: clarify and expand description of --signoff

Building on past documentation improvements in b2c150d3aa (Expand
documentation describing --signoff, 2016-01-05), further clarify
that any project using Git may and often does set its own policy.

However, leave intact reference to the Linux DCO, which Git also
uses.  It is reasonable for Git to advocate for its own Signed-off-by
methodology in its documentation, as long as the documentation
remains respectful that YMMV and other projects may well have very
different contributor representations tied to Signed-off-by.

Signed-off-by: Bradley M. Kuhn <redacted>
Signed-off-by: Junio C Hamano <redacted>
Signed-off-by: Jeff King <redacted>
Reviewed-by: Taylor Blau <redacted>
Signed-off-by: Junio C Hamano <redacted>