Document @export_custom and additional range hints in GDScript exports

[Calinou]

This also adds a hint suffix example to C# exports (contributed by @ShawnHardern).

• See GDScript: Add @export_custom annotation godot#72912
  and tutorials/scripting/gdscript/gdscript_exports godot-docs-user-notes#143 (comment).

[tetrapod00]

FYI there's already an open PR for suffix hints #9785

[tetrapod00]

This might resolve #9925 - note that the OP is asking for a specific example in addition to generally documenting export_custom.

[sighpher]

FYI there's already an open PR for suffix hints #9785

I think it looks like this PR better documents the feature so happy to close #9785 if that helps this PR move forward @Calinou @tetrapod00

[mhilbrunner]

Thanks, merged.

@ShawnHardern Thanks, that made the decision indeed easier. Apologies for not getting back to your original PR, sometimes its hard to keep up with the volume :/ In the future, feel free to ping people in your PR or ask on the #documentation channel on Rocketchat if you hear nothing back for a while. Shouödn't be on you of course, sadly sometimes things slip through. Thanks for contributing!

[mhilbrunner]

Cherrypicked to 4.3 in #10346.