Allow VisualScript nodes other than built-in methods and members to have descriptions

[Jummit]

Godot version: 3.1

Issue description: Currently only built-in methods and members can have descriptions. It would be useful if VisualScript specific nodes like the flow control or the basic type manipulation could have descriptions too. There should be a way to change the description of a custom visualscript node, too. A _get_description function would be great.

[swarnimarun]

Aah I had wanted to do this. But AFAIK adding runtime element docs is problematic.

I will probably do it with the tooltip(hints) feature I had planned.