Clarify using underscores to denote private variables

**Your Godot version:**
3.3

**Issue description:**
The documentation states

> Prepend a single underscore (_) to virtual methods functions the user must override, private functions, and private variables

However, this doesn't mention anything about private Constants and Enums. What should one do for them? What about export variables? Can they be private?

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/latest/tutorials/scripting/gdscript/gdscript_styleguide.html