Document trio.from_thread.check_cancelled

Author: richardsheridan

docs/source/reference-core.rst

@@ -1820,6 +1820,11 @@ to spawn a child thread, and then use a :ref:`memory channel
 
 .. literalinclude:: reference-core/from-thread-example.py
 
+You can also perform a non-blocking check for cancellation from threads spawned
+by func:`trio.to_thread.run_sync`.
+
+.. autofunction:: trio.from_thread.check_cancelled
+
 Threads and task-local storage
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~