Allow documentation of exported script properties

[endragor]

Often script properties are supposed to be edited by artists (or, in general, not by the author of the script) and a tooltip explaining their meaning would be helpful. But it seems currently Godot only supports tooltips for builtin properties documented in classes.xml. It should be easy to expand PropertyInfo with documentation field which could be settable from GDScript or other bindings.

[Zylann]

Duplicate of #6204