docs: describe behavior of relative submodule URLs

Since the relative submodule URLs have been introduced in f31a522a2d, they
do not conform to the rules for resolving relative URIs but rather to
those of relative directories.

Document that behavior.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-submodule.txt

@@ -79,7 +79,12 @@ to exist in the superproject. If <path> is not given, the
 <repository> is the URL of the new submodule's origin repository.
 This may be either an absolute URL, or (if it begins with ./
 or ../), the location relative to the superproject's origin
-repository. If the superproject doesn't have an origin configured
+repository (Please note that to specify a repository 'foo.git'
+which is located right next to a superproject 'bar.git', you'll
+have to use '../foo.git' instead of './foo.git' - as one might expect
+when following the rules for relative URLs - because the evaluation
+of relative URLs in Git is identical to that of relative directories).
+If the superproject doesn't have an origin configured
 the superproject is its own authoritative upstream and the current
 working directory is used instead.
 +