docs: specifically mention "OpenPMIx" (vs. "PMIx")

Also update the URL to point to the OpenPMIx RTD docs, and add a
clarification that we used the term "PMIx" heavily throughout the rest
of the docs to refer to "OpenPMIx".

Signed-off-by: Jeff Squyres <[email protected]>
(cherry picked from commit 883030f)

docs/installing-open-mpi/required-support-libraries.rst

@@ -74,24 +74,30 @@ system.
 
     .. note:: The Open MPI community has heavily tested Libevent
               |event_embedded_version|.  Other versions should *work*,
-              but |mdash| unlike Hwloc, Open PMIx, and PRRTE |mdash|
+              but |mdash| unlike Hwloc, OpenPMIx, and PRRTE |mdash|
               there is not much reason to upgrade to use a later
               version of Libevent.
 
-* `PMIx <https://pmix.org/>`_
+* `OpenPMIx <https://docs.openpmix.org/>`_
 
   * This library is required; Open MPI will not build without it.
   * **Minimum version required when building without PRRTE:**
     |pmix_min_version|
 
-    .. note:: While the minimum version of Open PMIx will *work*, it
+    .. note:: While the minimum version of OpenPMIx will *work*, it
               may still have bugs and/or have less functionality as
               compared to later versions.
 
               The Open MPI community generally recomends using the
-              latest available version of Open PMIx unless there is a
+              latest available version of OpenPMIx unless there is a
               specific reason not to.
 
+    .. note:: While `OpenPMIx <https://docs.openpmix.org/>`_ is the
+              formal name of the software that implements the `PMIx
+              <https://pmix.org/>`_ standard, the term "PMIx" is used
+              extensively throughout this documentation to refer to
+              the OpenPMIx software package.
+
   * **Minimum version required when building with PRRTE:** `See the
     PRRTE project documentation <https://docs.prrte.org/>`_.
   * **Version embedded in Open MPI distribution:**