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 <bkuhn@sfconservancy.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/signoff-option.txt

@@ -3,12 +3,16 @@ ifdef::git-commit[]
 endif::git-commit[]
 --signoff::
 --no-signoff::
-	Add Signed-off-by line by the committer at the end of the commit
-	log message.  The meaning of a signoff depends on the project,
-	but it typically certifies that committer has
-	the rights to submit this work under the same license and
-	agrees to a Developer Certificate of Origin
-	(see http://developercertificate.org/ for more information).
+	Add a Signed-off-by trailer by the committer at the end of the commit
+	log message.  The meaning of a signoff depends on the project
+	to which you're committing.  For example, it may certify that
+	the committer has the rights to submit the work under the
+	project's license or agrees to some contributor representation,
+	such as a Developer Certificate of Origin.
+	(See http://developercertificate.org for the one used by the
+	Linux kernel and Git projects.)  Consult the documentation or
+	leadership of the project to which you're contributing to
+	understand how the signoffs are used in that project.
 +
 The --no-signoff option can be used to countermand an earlier --signoff
 option on the command line.