Add info about layering properties

docs/docsite/rst/dev_guide/developing_modules_documenting.rst

@@ -287,7 +287,7 @@ If you're writing multiple related modules, they may share common documentation,
 
 Modules should only use items from a doc fragment if the module will implement all of the interface documented there in a manner that behaves the same as the existing modules which import that fragment. The goal is that items imported from the doc fragment will behave identically when used in another module that imports the doc fragment.
 
-By default, only the ``DOCUMENTATION`` property from a doc fragment is inserted into the module documentation. It is possible to define additional properties in the doc fragment in order to import only certain parts of a doc fragment or mix and match as appropriate.
+By default, only the ``DOCUMENTATION`` property from a doc fragment is inserted into the module documentation. It is possible to define additional properties in the doc fragment in order to import only certain parts of a doc fragment or mix and match as appropriate. If a property is defined in both the doc fragment and the module, the module value overrides the doc fragment.
 
 Here is an example doc fragment named ``example_fragment.py``: