C#: Requirement of `partial` is not explained or documented

**Your Godot version:**
v4.1.1.stable.mono.official [bd6af8e0e]
**Issue description:**
Currently, if `partial` is omitted from a Node-inherited class definition, it does emit the error `GD0001: Missing partial modifier on declaration of type 'PlayerControllerState' which is a subclass of 'Godot.GodotObject'`. However, nothing in the documentation explains why this is, or even alerts the user that `partial` is required when making a Node script. I ended up finding the reasoning in [a blog post](https://godotengine.org/article/whats-new-in-csharp-for-godot-4-0/#engine-interop-with-source-generators) that I only found through a link on [the (currently defunct) Godot ask site](https://ask.godotengine.org/148970/c%23-version-build-failed).
**URL to the documentation page:**
https://docs.godotengine.org/en/stable/tutorials/scripting/c_sharp/c_sharp_basics.html
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