Document 110239: How callbacks work on Operator properties

After writing add-ons for like the past 8 years, I didn't realize that the first parameter of update callbacks of operator properties, conventionally named `self` is not actually what a Python coder would expect from a variable named `self`. I tried to document that here.

This is in response to [what I thought was a bug](https://projects.blender.org/blender/blender/issues/110239).

Pull Request: https://projects.blender.org/blender/blender/pulls/110242

doc/python_api/examples/bpy.props.4.py

@@ -10,6 +10,12 @@ All properties define update functions except for CollectionProperty.
 .. warning::
    Remember that these callbacks may be executed in threaded context.
 
+.. warning::
+   If the property belongs to an Operator, the update callback's first 
+   parameter will be an OperatorProperties instance, rather than an instance 
+   of the operator itself. This means you can't access other internal functions
+   of the operator, only its other properties.
+
 """
 
 import bpy