gitmodules: clarify what history depth a shallow clone has

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitmodules.txt

@@ -84,8 +84,8 @@ submodule.<name>.ignore::
 
 submodule.<name>.shallow::
 	When set to true, a clone of this submodule will be performed as a
-	shallow clone unless the user explicitly asks for a non-shallow
-	clone.
+	shallow clone (with a history depth of 1) unless the user explicitly
+	asks for a non-shallow clone.
 
 
 EXAMPLES