Note on branch-specific labels with custom rules.pl

Clarify in the documentation that when a project has a custom
rules.pl defined or inherited the branch-specific filtering of
a label has no effect.

Change-Id: I3b5a8e2a17b3f3c615bda1290ce3bfa5b58850bc

Documentation/config-labels.txt

diff --git a/Documentation/config-labels.txt b/Documentation/config-labels.txt
index e1a25ca..e4b9a83 100644
--- a/Documentation/config-labels.txt
+++ b/Documentation/config-labels.txt
@@ -364,6 +364,8 @@
 parts in `project.config` file. That means from the UI a user can always
 assign permissions for that label on a branch, but this permission is then
 ignored if the label doesn't apply for that branch.
+Additionally, the `branch` modifier has no effect when the submit rule
+is customized in the rules.pl of the project or inherited from parent projects.
 
 [[label_example]]
 === Example