Improve documentation on creating editor export plugins

**Your Godot version:** 3.2.3

**Issue description:**

I am interested in making plugins for Godot with gdscript and I found good knowledge on it at Godot docs but one thing that's bothering me is that I want to create a export plugin I found function reference but it doesn't make any sense because I am still confuse on how to do so can anyone one assist me

**URL to the documentation page:**