Add self field to all method signatures in module api docs

Add a `self` parameter to each method signature in the module api
documentation. This makes them easier to copy-paste, and more accurately
conveys that Synapse expects them to be methods, rather than static
functions.

docs/modules/writing_a_module.md

@@ -48,7 +48,11 @@ Modules can register web resources onto Synapse's web server using the following
 API method:
 
 ```python
-def ModuleApi.register_web_resource(path: str, resource: IResource) -> None
+def ModuleApi.register_web_resource(
+    self,
+    path: str,
+    resource: IResource,
+) -> None
 ```
 
 The path is the full absolute path to register the resource at. For example, if you