Advanced exports section is missing crucial information

**Your Godot version:**
master

**Issue description:**
There's an Advanced Exports section which doesn't explain anything about advanced exports. Needs to be written?
Proposal: should include examples of how to export custom properties / how to use _get_property_list(), details about the PropertyHint enum, grouping, etc.

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