The GDScript Exports docs need to cover `property_can_revert` and `property_get_revert` too.

**Your Godot version:**
3.2 stable

**Issue description:**
If you create runtime export variables via the `_get`, `_set`, and `_get_property_list` methods, then there is no clear way to define a default value for reverting any of the generated properties. The Inspector doesn't know that it can revert them.

The Inspector does provide overrides for forcibly determining true/false if a property can be overridden and a Variant response for what the reverted value should be via the `property_can_revert` and `property_get_revert` functions, but these methods are undocumented. I have covered their exclusion from the documentation in godotengine/godot#43078.

**URL to the documentation page (if already existing):**
https://docs.godotengine.org/en/stable/getting_started/scripting/gdscript/gdscript_exports.html#advanced-exports