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>

Documentation/git-am.txt

@@ -35,6 +35,7 @@ OPTIONS
 --signoff::
 	Add a `Signed-off-by:` line to the commit message, using
 	the committer identity of yourself.
+	See the signoff option in linkgit:git-commit[1] for more information.
 
 -k::
 --keep::

Documentation/git-cherry-pick.txt

@@ -100,6 +100,7 @@ effect to your index in a row.
 -s::
 --signoff::
 	Add Signed-off-by line at the end of the commit message.
+	See the signoff option in linkgit:git-commit[1] for more information.
 
 -S[<keyid>]::
 --gpg-sign[=<keyid>]::

Documentation/git-commit.txt

@@ -154,7 +154,11 @@ OPTIONS
 -s::
 --signoff::
 	Add Signed-off-by line by the committer at the end of the commit
-	log message.
+	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).
 
 -n::
 --no-verify::

Documentation/git-format-patch.txt

@@ -109,6 +109,7 @@ include::diff-options.txt[]
 --signoff::
 	Add `Signed-off-by:` line to the commit message, using
 	the committer identity of yourself.
+	See the signoff option in linkgit:git-commit[1] for more information.
 
 --stdout::
 	Print all commits to the standard output in mbox format,

Documentation/git-revert.txt

@@ -89,6 +89,7 @@ effect to your index in a row.
 -s::
 --signoff::
 	Add Signed-off-by line at the end of the commit message.
+	See the signoff option in linkgit:git-commit[1] for more information.
 
 --strategy=<strategy>::
 	Use the given merge strategy.  Should only be used once.