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.5.py

@@ -6,6 +6,10 @@ Getter/setter functions can be used for boolean, int, float, string and enum pro
 If these callbacks are defined the property will not be stored in the ID properties
 automatically. Instead, the `get` and `set` functions will be called when the property
 is respectively read or written from the API.
+
+.. warning::
+   Remember that these callbacks may be executed in threaded context.
+
 """
 import bpy