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>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: gitster

Documentation/gitfaq.adoc

@@ -83,6 +83,21 @@ 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?::
+	As it makes it harder to argue against one who tells the court
+	"the log message of the commit ends with a `Signed-off-by`
+	trailer by person X, but it is very plausible that it was done
+	by inertia without person X really intending to certify what
+	DCO says, hence the `Signed-off-by` trailer is meaningless", if
+	we add more publicized ways to add sign-off automatically. Git
+	does not, and will not, have a configuration variable to
+	enable the `--signoff` command line option it by default.
++
+There exists `format.signoff`, but that is a historical mistake, and
+it is not an excuse to pile on more mistakes of the same kind on top.
+
+
 Credentials
 -----------
 

Documentation/signoff-option.adoc

@@ -16,3 +16,15 @@ endif::git-commit[]
 +
 The `--no-signoff` option can be used to countermand an earlier `--signoff`
 option on the command line.
++
+As it makes it harder to argue against one who tells the court "the
+log message of the commit ends with a `Signed-off-by` trailer by person
+X, but it is very plausible that it was done by inertia without person
+X really intending to certify what DCO says, hence the `Signed-off-by`
+trailer is meaningless", if we add more publicized ways to add
+sign-off automatically. Git does not, and will not, have a
+configuration variable to enable the `--signoff` command line option
+it by default.
++
+There exists `format.signoff`, but that is a historical mistake, and
+it is not an excuse to pile on more mistakes of the same kind on top.