bpo-37703: improve asyncio.gather documentation regarding cancellation (GH-15312)

These changes updates the doc to comprehensively mention the behaviour of gather.cancel()

Automerge-Triggered-By: @asvetlov

Doc/library/asyncio-task.rst

@@ -387,6 +387,14 @@ Running Tasks Concurrently
       #     Task C: Compute factorial(4)...
       #     Task C: factorial(4) = 24
 
+   .. note::
+      If *return_exceptions* is False, cancelling gather() after it
+      has been marked done won't cancel any submitted awaitables.
+      For instance, gather can be marked done after propagating an
+      exception to the caller, therefore, calling ``gather.cancel()``
+      after catching an exception (raised by one of the awaitables) from
+      gather won't cancel any other awaitables.
+
    .. versionchanged:: 3.7
       If the *gather* itself is cancelled, the cancellation is
       propagated regardless of *return_exceptions*.

Lib/asyncio/tasks.py

@@ -729,6 +729,13 @@ def gather(*coros_or_futures, loop=None, return_exceptions=False):
     the outer Future is *not* cancelled in this case.  (This is to
     prevent the cancellation of one child to cause other children to
     be cancelled.)
+
+    If *return_exceptions* is False, cancelling gather() after it
+    has been marked done won't cancel any submitted awaitables.
+    For instance, gather can be marked done after propagating an
+    exception to the caller, therefore, calling ``gather.cancel()``
+    after catching an exception (raised by one of the awaitables) from
+    gather won't cancel any other awaitables.
     """
     if not coros_or_futures:
         if loop is None:

Misc/NEWS.d/next/Documentation/2019-08-16-20-25-42.bpo-37703.Qm_l_H.rst

@@ -0,0 +1,2 @@
+Updated Documentation to comprehensively elaborate on the behaviour of
+gather.cancel()