[doc] clarify ClientTimeout behavior (#6683) (#6685)

* [doc] clarify ClientTimeout behavior

be clear that the 5 second timeout is also rounded. see: https://github.com/aio-libs/aiohttp/blob/master/aiohttp/helpers.py#L638

* Update docs/client_reference.rst

Co-authored-by: Sam Bull <aa6bs0@sambull.org>

Co-authored-by: Sam Bull <aa6bs0@sambull.org>
(cherry picked from commit 90964d0)

Co-authored-by: Rafael Stahl <dummdoof-doof@web.de>

docs/client_reference.rst

@@ -1713,7 +1713,7 @@ ClientTimeout
 
    .. note::
 
-      Timeouts larger than 5 seconds are rounded for scheduling on the next
+      Timeouts of 5 seconds or more are rounded for scheduling on the next
       second boundary (an absolute time where microseconds part is zero) for the
       sake of performance.