Add a warning about C# differences in classref doc

[raulsntos]

Adds a warning to the class reference documentation for classes that have notable[^1] differences between GDScript and C#.

• Closes https://github.com/godotengine/godot/pull/70852 as superseded.
  • The warning added by this PR + https://github.com/godotengine/godot-docs/pull/6679 seem sufficient to supersed.
• Closes https://github.com/godotengine/godot-docs/issues/5806.

[^1]: By notable I mean important enough to be mentioned explicitly in the C# API differences to GDScript documentation page.

[akien-mga]

Would it be worth extending EditorHelp to do the same? (Possibly only when MODULE_MONO_ENABLED)

[raulsntos]

I think so yes. Unfortunately we don't support [note] in EditorHelp (yet^1).

[YuriSizov]

Thanks!