Docs: Note that C# does not have "copy constructors" for variant types

[tetrapod00]

Closes godotengine/godot-docs#10148.

This PR notes that constructors of the form public Basis(Basis from) are not available in C#.

To create a copy of a struct variant type in C#, the following syntax with a "copy constructor" is not available:

Basis original = Basis.Identity;
Basis copy = new Basis(original);

Instead, the following syntax can be used:

Basis original = Basis.Identity;
Basis copy = original;

[paulloz]

This is fine from the C# side. Although, I'm personally not convinced how useful it is, given this is very standard struct behaviour.

[AThousandShips]

I think this might not be the appropriate place to put this, while we do have suggested changes like #98499 this is very much a part of the general differences with C# and I think belongs in the dedicated C# differences page

[tetrapod00]

Yeah, I do think this change is pretty borderline. If someone didn't go to the effort of making a docs issue, I wouldn't have made this PR. But clearly existing users do use the online class ref for C#, and they are confused by the differences in API when they exist and are not documented.

[Mickeon]

I'm skeptical on adding this, as all of these repetitive notes are a burden to maintain and localize. Although there are precedents to this (which I also don't like, personally).

Obviously I do think it has a place in the class reference, but perhaps it should be an automatically generated note, instead. That would make it similar to #85207 , which also originally started as a spam of notes.

[tetrapod00]

I made the changes, so if this should be merged it can be. But I think we're deferring this until it's implemented as an automatically generated note, to reduce future maintenance?

In the meantime I'll look into a smaller docs change on the C# API differences manual page.