Docs: formalize naming for generic callbacks in BKE_callbacks.h

Add a doc-string explaining the purpose of each call back and
how they should be used.

Also add a currently unused callback 'POST_FAIL' that is to be used in
cases the action fails - giving script authors, a guarantee that a
call to `pre` will always have a matching `post/post_fail` call.

- D11422: adds a callback that can use 'post_fail'.
- T88696: proposed these conventions.

source/blender/python/intern/bpy_app_handlers.c

@@ -43,6 +43,9 @@ void bpy_app_generic_callback(struct Main *main,
 
 static PyTypeObject BlenderAppCbType;
 
+/**
+ * See `BKE_callbacks.h` #eCbEvent declaration for the policy on naming.
+ */
 static PyStructSequence_Field app_cb_info_fields[] = {
     {"frame_change_pre",
      "Called after frame change for playback and rendering, before any data is evaluated for the "