Clarify how to use MultiMesh.set_instance_color.

[rcorre]

Just calling set_instance_color will do nothing unless you have set
color_format and vertex_color_use_as_albedo. This is really confusing,
and I only discovered my error by finding
godotengine/godot#10217 from another confused
user.

The docs should call out these requirements.

[akien-mga]

See that file's header:

.. Generated automatically by doc/tools/makerst.py in Godot's source tree.
.. DO NOT EDIT THIS FILE, but the MultiMesh.xml source instead.
.. The source is found in doc/classes or modules/<name>/doc_classes.

This should be changed in https://github.com/godotengine/godot