docs: Link to MPI Forum docs web page

Instead of having hard-coded references to old versions of the MPI
standard, link out to the MPI Forum web site docs page, and update the
cited section of the doc to be something that exists in both the older
versions of the MPI standard and also in the current (MPI-4.1)
standard.

Signed-off-by: Jeff Squyres <jeff@squyres.com>

Author: jsquyres

docs/man-openmpi/man3/MPI_Recv.3.rst

@@ -74,8 +74,9 @@ after the receive buffer contains the newly received message. A receive
 can complete before the matching send has completed (of course, it can
 complete only after the matching send has started).
 
-The blocking semantics of this call are described in Section 3.4 of the
-MPI-1 Standard, "Communication Modes."
+The blocking semantics of this call are described in the
+"Communication Modes" section of the `MPI Standard
+<https://www.mpi-forum.org/docs/>`_.
 
 The receive buffer contains a number (defined by the value of *count*)
 of consecutive elements. The first element in the set of elements is
@@ -99,10 +100,11 @@ The *count* argument indicates the maximum number of entries of type
 received, use the :ref:`MPI_Get_count` function to determine the actual number
 of entries within that message.
 
-To receive messages of unknown length, use the :ref:`MPI_Probe` function. (For
-more information about :ref:`MPI_Probe` and :ref:`MPI_Cancel`, see their respective
-man pages; also, see Section 3.8 of the MPI-1 Standard, "Probe and
-Cancel.")
+To receive messages of unknown length, use the :ref:`MPI_Probe`
+function.  For more information about :ref:`MPI_Probe` and
+:ref:`MPI_Cancel`, see their respective man pages and the "Probe and
+Cancel" section of the `MPI Standard
+<https://www.mpi-forum.org/docs/>`_.
 
 A message can be received by a receive operation only if it is addressed
 to the receiving process, and if its source, tag, and communicator
@@ -133,8 +135,9 @@ by the receiver).
 Source = destination is allowed, that is, a process can send a message
 to itself. However, it is not recommended for a process to send messages
 to itself using the blocking send and receive operations described
-above, since this may lead to deadlock. See Section 3.5 of the MPI-1
-Standard, "Semantics of Point-to-Point Communication."
+above, since this may lead to deadlock.  See the "Semantics of
+Point-to-Point Communication" of the `MPI Standard
+<https://www.mpi-forum.org/docs/>`_ for more details.
 
 If your application does not need to examine the *status* field, you can
 save resources by using the predefined constant ``MPI_STATUS_IGNORE`` as a