config doc: user.signingkey is also used for signed commits
config doc: user.signingkey is also used for signed commits

The description of the user.signingkey option only mentioned its use
when creating a signed tag. Make it clear that is is also used when
creating signed commits.

Signed-off-by: Nicolas Vigier <dac6cef728d8800bf24e6590a30175f001dfb476@mars-attacks.org>
Signed-off-by: Jonathan Nieder <b57189c5f2fd5b3daf3350a77fde90def1080fa5@gmail.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git