docs: there is no such thing as "libmpi-mt".

Clarify wording; we're really just talking about multi-threaded
applications.

Signed-off-by: Jeff Squyres <[email protected]>
(cherry picked from commit a1b40a4)

Author: jsquyres

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

@@ -109,8 +109,9 @@ it is received.
 NOTE
 ----
 
-Users of libmpi-mt should remember that two threads may do an :ref:`MPI_Iprobe`
-that actually returns true for the same message for both threads.
+Multi-threaded application developers should remember that two threads
+calling :ref:`MPI_Iprobe` may return true for the same message in both
+threads.
 
 
 ERRORS