commit: document that $command.signoff will not be added

Every now and then we see this coming up on the list.  Let's help
new contributors who are not aware of past discussions by clearly
documenting our past consensus.

Helped-by: brian m. carlson <sandals@crustytoothpaste.net>
Helped-by: Elijah Newren <newren@gmail.com>
Helped-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitfaq.adoc

@@ -83,6 +83,25 @@ Windows would be the configuration `"C:\Program Files\Vim\gvim.exe" --nofork`,
 which quotes the filename with spaces and specifies the `--nofork` option to
 avoid backgrounding the process.
 
+[[sign-off]]
+Why not have `commit.signoff` and other configuration variables?::
+	Git intentionally does not (and will not) provide a
+	configuration variable, such as `commit.signoff`, to
+	automatically add `--signoff` by default.  The reason is to
+	protect the legal and intentional significance of a sign-off.
+	If there were more automated and widely publicized ways for
+	sign-offs to be appended, it would become easier for someone
+	to argue later that a "Signed-off-by" trailer was just added
+	out of habit or by automation, without the committer's full
+	awareness or intent to certify their agreement with the
+	Developer Certificate of Origin (DCO) or a similar statement.
+	This could undermine the sign-off’s credibility in legal or
+	contractual situations.
++
+There exists `format.signoff`, but that is a historical mistake, and
+it is not an excuse to add more mistakes of the same kind on top.
+
+
 Credentials
 -----------
 

Documentation/signoff-option.adoc

@@ -16,3 +16,7 @@ endif::git-commit[]
 +
 The `--no-signoff` option can be used to countermand an earlier `--signoff`
 option on the command line.
++
+Git does not (and will not) have a configuration variable to enable
+the `--signoff` command line option by default; see the
+`commit.signoff` entry in the gitfaq for more details.