gh-98641: Document difference between task group and gather (#103644)

The purpose of the comments is to rule out the implication that asyncio.TaskGroup is a drop-in replacement / better alternative to asyncio.gather().

Author: why-not-try-calmer

Doc/library/asyncio-task.rst

@@ -256,8 +256,9 @@ Creating Tasks
 
    .. note::
 
-      :meth:`asyncio.TaskGroup.create_task` is a newer alternative
-      that allows for convenient waiting for a group of related tasks.
+      :meth:`asyncio.TaskGroup.create_task` is a new alternative
+      leveraging structural concurrency; it allows for waiting
+      for a group of related tasks with strong safety guarantees.
 
    .. important::
 
@@ -340,7 +341,7 @@ Example::
         async with asyncio.TaskGroup() as tg:
             task1 = tg.create_task(some_coro(...))
             task2 = tg.create_task(another_coro(...))
-        print("Both tasks have completed now.")
+        print(f"Both tasks have completed now: {task1.result()}, {task2.result()}")
 
 The ``async with`` statement will wait for all tasks in the group to finish.
 While waiting, new tasks may still be added to the group
@@ -459,8 +460,12 @@ Running Tasks Concurrently
    Tasks/Futures to be cancelled.
 
    .. note::
-      A more modern way to create and run tasks concurrently and
-      wait for their completion is :class:`asyncio.TaskGroup`.
+      A new alternative to create and run tasks concurrently and
+      wait for their completion is :class:`asyncio.TaskGroup`. *TaskGroup*
+      provides stronger safety guarantees than *gather* for scheduling a nesting of subtasks:
+      if a task (or a subtask, a task scheduled by a task)
+      raises an exception, *TaskGroup* will, while *gather* will not,
+      cancel the remaining scheduled tasks).
 
    .. _asyncio_example_gather: