Add some small advise about module's update

This seems pretty obvious (after you know it) but I learned it
the hard way after struggling for half an hour.

doc/Language/modules.pod6

@@ -701,6 +701,9 @@ module directly from the module folder you just created.
 zef install ./your-module-folder
 =end code
 
+Be advised that you must uninstall and install the given module for the latest
+changes to take place.
+
 =end item
 
 =head2 Upload your Module to CPAN