Tutorial: Impove section on Lock primitive.

Author: peterhinch

v3/docs/TUTORIAL.md

@@ -627,17 +627,28 @@ This guarantees unique access to a shared resource. In the following code
 sample a `Lock` instance `lock` has been created and is passed to all tasks
 wishing to access the shared resource. Each task attempts to acquire the lock,
 pausing execution until it succeeds.
+```python
+from asyncio import Lock
+lock = Lock()
+```
+Synchronous methods:
+ * `locked` No args. Returns `True` if locked.
+ * `release` No args. Releases the lock. See note below.
+Asynchronous method:
+ * `acquire` No args. Pauses until the lock has been acquired. Use by executing
+ `await lock.acquire()`.
 
+A task waiting on a lock may be cancelled or may be run subject to a timeout.
+The normal way to use a `Lock` is in a context manager:
 ```python
 import asyncio
 from asyncio import Lock
 
 async def task(i, lock):
     while 1:
-        await lock.acquire()
-        print("Acquired lock in task", i)
-        await asyncio.sleep(0.5)
-        lock.release()
+        async with lock:
+            print("Acquired lock in task", i)
+            await asyncio.sleep(0.5)
 
 async def main():
     lock = Lock()  # The Lock instance
@@ -648,26 +659,24 @@ async def main():
 
 asyncio.run(main())  # Run for 10s
 ```
+Use of a context manager is strongly recommended - otherwise an application must
+ensure that `.release` is only ever called when that same task has called
+`.locked`. Calling `.release` on an unlocked `Lock` will raise a `ValueError`.
+Calling it on a `Lock` which has been locked by another task will cause that
+second task to produce a `ValueError` when it attempts to release the `Lock` or
+when its context manager exits. Context managers avoid these issues.
 
-Methods:
-
- * `locked` No args. Returns `True` if locked.
- * `release` No args. Releases the lock.
- * `acquire` No args. Coro which pauses until the lock has been acquired. Use
- by executing `await lock.acquire()`.
-
-A task waiting on a lock may be cancelled or may be run subject to a timeout.
-The normal way to use a `Lock` is in a context manager:
-
+For the brave the following illustrates use without a CM.
 ```python
 import asyncio
 from asyncio import Lock
 
 async def task(i, lock):
     while 1:
-        async with lock:
-            print("Acquired lock in task", i)
-            await asyncio.sleep(0.5)
+        await lock.acquire()
+        print("Acquired lock in task", i)
+        await asyncio.sleep(0.5)
+        lock.release()
 
 async def main():
     lock = Lock()  # The Lock instance
@@ -678,7 +687,6 @@ async def main():
 
 asyncio.run(main())  # Run for 10s
 ```
-
 ###### [Contents](./TUTORIAL.md#contents)
 
 ## 3.2 Event