Py API doc: Add warning that py-defined property accessor callbacks may be called in threaded context.

At least Depsgraph evaluation and liboverride diffing do process IDs in
parallel, so python code in py-defined properties should not access any
data outside of their owner ID.

Ref. T100203.

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

@@ -6,6 +6,10 @@ It can be useful to perform an action when a property is changed and can be
 used to update other properties or synchronize with external data.
 
 All properties define update functions except for CollectionProperty.
+
+.. warning::
+   Remember that these callbacks may be executed in threaded context.
+
 """
 
 import bpy