docs: add some missing text to MPI_Mrecv/Imrecv

Add text about status to the MPI_Mrecv.3/MPI_Imrecv.3 man pages that
exists in other receive-like functions.

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

Author: jsquyres

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

@@ -90,6 +90,10 @@ canceling the matched message receive would violate MPI message ordering
 rules (e.g., if another message matching the same message signature has
 matched |mdash| and possibly received |mdash| before this :ref:`MPI_Imrecv` is canceled).
 
+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 special value for the *status* argument.
+
 
 ERRORS
 ------

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

@@ -75,6 +75,10 @@ the call returns immediately with the *status* object set to *source* =
 ``MPI_PROC_NULL``, *tag* = ``MPI_ANY_TAG``, and *count* = 0, as if a receive
 from ``MPI_PROC_NULL`` was issued.
 
+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 special value for the *status* argument.
+
 
 ERRORS
 ------