C# Preprocessor defines doc is out of date

[ace24713]

Your Godot version:
3.5.1, but issue is present on latest too

Issue description:
Documentation on C# Preprocessor defines is no longer accurate, some of the described features were removed:
see godotengine/godot#40595
and the 3.x counterpart godotengine/godot#41408

and possibly relevant, the not-yet-merged code that would bring some of those features back:
here godotengine/godot#53920
and the 3.x counterpart godotengine/godot#53921

Reading the material, it's still unclear to me if these features will ever come back in any form, but at least the documentation being up to date would've saved me a few hours of debugging.

URL to the documentation page:
https://docs.godotengine.org/en/stable/tutorials/scripting/c_sharp/c_sharp_features.html#preprocessor-defines