docs/library/uasyncio.rst: Add docs for ThreadSafeFlag.

Signed-off-by: Jim Mussared <jim.mussared@gmail.com>

docs/library/uasyncio.rst

@@ -125,6 +125,9 @@ class Event
 
     Set the event.  Any tasks waiting on the event will be scheduled to run.
 
+    Note: This must be called from within a task. It is not safe to call this
+    from an IRQ, scheduler callback, or other thread. See `ThreadSafeFlag`.
+
 .. method:: Event.clear()
 
     Clear the event.
@@ -136,6 +139,29 @@ class Event
 
     This is a coroutine.
 
+class ThreadSafeFlag
+--------------------
+
+.. class:: ThreadSafeFlag()
+
+    Create a new flag which can be used to synchronise a task with code running
+    outside the asyncio loop, such as other threads, IRQs, or scheduler
+    callbacks.  Flags start in the cleared state.
+
+.. method:: ThreadSafeFlag.set()
+
+    Set the flag.  If there is a task waiting on the event, it will be scheduled
+    to run.
+
+.. method:: ThreadSafeFlag.wait()
+
+    Wait for the flag to be set.  If the flag is already set then it returns
+    immediately.
+
+    A flag may only be waited on by a single task at a time.
+
+    This is a coroutine.
+
 class Lock
 ----------