Mention that FBX support is partial in List of features

Support for animated FBX scenes is techically present, but it's
usually too broken to be usable in production.

For reference, this partial support is already mentioned on the FAQ.

about/list_of_features.rst

@@ -404,7 +404,7 @@ Import
    - glTF 2.0 *(recommended)*.
    - `ESCN <https://github.com/godotengine/godot-blender-exporter>`__
      (direct export from Blender).
-   - FBX.
+   - FBX (static meshes only).
    - Collada (.dae).
    - Wavefront OBJ (static scenes only, can be loaded directly as a mesh).