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 <boklm@mars-attacks.org>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Merge branch 'nv/doc-config-signingkey'

* nv/doc-config-signingkey:
  config doc: user.signingkey is also used for signed commits
CodingGuidelines: style for multi-line comments

The style for multi-line comments is often mentioned and should be documented
for clarity.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>