documentation: improve submodule.<name>.{url, path} description

documentation: improve submodule.<name>.{url, path} description

Unlike the url variable a user cannot override the the path variable,
as it is part of the content together with the gitlink at the given
path. To avoid confusion do not mention the .path variable in the config
section and rely on the documentation provided in gitmodules[5].

Enhance the description of submodule.<name>.url and mention its two use
cases separately.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>