MultiMesh resource buffer description

**Your Godot version:**
4.1-dev1

**Issue description:**
The MultiMesh "buffer" property currently contains no description, while the description from RenderingServer::multimesh_set_buffer would work exactly as needed, or at least something that says "see RenderingServer::multimesh_set_buffer and RenderingServer::multimesh_get_buffer" for more information
https://docs.godotengine.org/en/stable/classes/class_renderingserver.html#class-renderingserver-method-multimesh-set-buffer

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/latest/classes/class_multimesh.html#class-multimesh-property-buffer