submodule-config: clarify parsing of null_sha1 element

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

Author: stefanbeller

Documentation/technical/api-submodule-config.txt

@@ -56,8 +56,11 @@ Functions
 
 	The same as above but lookup by name.
 
-If given the null_sha1 as treeish_name the local configuration of a
-submodule will be returned (e.g. consolidated values from local git
+Whenever a submodule configuration is parsed in `parse_submodule_config_option`
+via e.g. `gitmodules_config()`, it will overwrite the null_sha1 entry.
+So in the normal case, when HEAD:.gitmodules is parsed first and then overlayed
+with the repository configuration, the null_sha1 entry contains the local
+configuration of a submodule (e.g. consolidated values from local git
 configuration and the .gitmodules file in the worktree).
 
 For an example usage see test-submodule-config.c.