Improve the modules doc

docs/modules/writing_a_module.md

@@ -64,4 +64,7 @@ callbacks for.
 Modules can register callbacks using one of the module API's `register_[...]_callbacks`
 methods. The callback functions are passed to these methods as keyword arguments, with
 the callback name as the argument name and the function as its value. This is demonstrated
-in the example below. A `register_[...]_callbacks` method exists for each category.
+in the example below. A `register_[...]_callbacks` method exists for each category.
+
+Callbacks for each category can be found by clicking the relevant links in the navigation
+panel on the left-hand side of this documentation.