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>

Documentation/config.txt

@@ -2706,12 +2706,13 @@ stash.showStat::
 	option will show diffstat of the stash.  Defaults to true.
 	See description of 'show' command in linkgit:git-stash[1].
 
-submodule.<name>.path::
 submodule.<name>.url::
-	The path within this project and URL for a submodule. These
-	variables are initially populated by 'git submodule init'. See
-	linkgit:git-submodule[1] and linkgit:gitmodules[5] for
-	details.
+	The URL for a submodule. This variable is copied from the .gitmodules
+	file to the git config via 'git submodule init'. The user can change
+	the configured URL before obtaining the submodule via 'git submodule
+	update'. After obtaining the submodule, the presence of this variable
+	is used as a sign whether the submodule is of interest to git commands.
+	See linkgit:git-submodule[1] and linkgit:gitmodules[5] for details.
 
 submodule.<name>.update::
 	The default update procedure for a submodule. This variable