MeshInstance.get_active_material() mentioned in docs, but does not exist in-engine.

[drwhut]

Your Godot version:
Godot 3.2.3 Stable

Issue description:
I've been working on a bug caused by materials for my project, and I noticed while looking at the documentation for the MeshInstance class, there was a method called get_active_material. I tried to use this method in my game, but an error came up saying it was not a method for the class - I then checked the in-editor documentation to see if it was mentioned there, and it wasn't.

URL to the documentation page:
MeshInstance.get_active_material

[YuriSizov]

This method was added in godotengine/godot#40313 and is part of the 3.2.4 release, which will soon be released as stable (you can try a release candidate for it). The docs are already synced, ahead of time.

[Calinou]

Duplicate of #4616 (same cause).