Add prefix policy to CONTRIBUTING.md

CONTRIBUTING.md

@@ -139,9 +139,10 @@ breaking points so your code is easy to read.
 
 ### Names
 
-Function and type names should be prefixed with `wlr_submodule_` (e.g. `struct
-wlr_drm_plane`, `wlr_output_set_cursor`).  For static functions and types local
-to a file, the names chosen aren't as important.
+Global function and type names should be prefixed with `wlr_submodule_` (e.g.
+`struct wlr_output`, `wlr_output_set_cursor`).  For static functions and
+types local to a file, the names chosen aren't as important.  Local function
+names shouldn't have a `wlr_` prefix.
 
 For include guards, use the header's filename relative to include.  Uppercase
 all of the characters, and replace any invalid characters with an underscore.