user-manual: add references to git-config man page
user-manual: add references to git-config man page

Direct editing of config files may be more natural for users than using
the git-config commandline; but we should still reference the
git-config man page when we describe such editing, so people know where
to go for details on the config file syntax and meanings of the
variables.

Signed-off-by: "J. Bruce Fields" <51738506c1b2ccb0761f23bdc612c93babf738ea@citi.umich.edu>

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