docs: fix outdated descriptions of -output-filename

contrib/completion/mpirun.zsh

@@ -179,7 +179,7 @@ _mpirun() {
 	'(-N -npernode --npernode -npersocket --npersocket)'{-N,-npernode,--npernode}'[Launch n processes per node on all allocated nodes]:npernode:_guard "[[\:digit\:]]#" "number"' \
 	'(-npernode --npernode -npersocket --npersocket)'{-npersocket,--npersocket}'[Launch n processes per socket on all allocated nodes]:npersocket:_guard "[[\:digit\:]]#" "number"' \
 	'(-ompi-server --ompi-server)'{-ompi-server,--ompi-server}'[Specify the URI of the Open MPI server, or the name of the file (specified as file:filename) that contains that info]:server uri:' \
-	'(-output-filename --output-filename)'{-output-filename,--output-filename}'[Redirect output from application processes into filename.rank]:filename:' \
+	'(-output-filename --output-filename)'{-output-filename,--output-filename}'[Redirect output from application processes into files under filename]:filename:' \
 	'(-output-proctable --output-proctable)'{-output-proctable,--output-proctable}'[Output the debugger proctable after launch]' \
 	'(-oversubscribe --oversubscribe -nooversubscribe --nooversubscribe)'{-oversubscribe,--oversubscribe}'[Nodes are allowed to be oversubscribed, even on a managed system]' \
 	'(-path --path)'{-path,--path}'[PATH to be used to look for executables to start processes]:path:_files -/' \

docs/man-openmpi/man1/mpirun.1.rst

@@ -363,17 +363,18 @@ For rankfiles:
 
 To manage standard I/O:
 
-* ``--output-filename <filename>``: Redirect the stdout, stderr, and
-  stddiag of all processes to a process-unique version of the
-  specified filename. Any directories in the filename will
-  automatically be created.  Each output file will consist of
-  ``filename.id``, where the ``id`` will be the processes' rank in
-  MPI_COMM_WORLD, left-filled with zero's for correct ordering in
-  listings. A relative path value will be converted to an absolute
-  path based on the cwd where mpirun is executed. Note that this will
-  not work on environments where the file system on compute nodes
-  differs from that where :ref:`mpirun(1) <man1-mpirun>` is
-  executed.
+* ``--output-filename <filename>``: Redirect output from application processes
+  into ``{filename}.prterun-{hostname}-{pid}@{jobid}.{rank}.(out|err|diag)``,
+  where ``{hostname}`` is the hostname, ``{pid}`` is the process ID of
+  ``mpirun``, ``{jobid}`` is the local job ID, and ``{rank}`` is the processes'
+  rank in ``MPI_COMM_WORLD``, left-filled with zero's for correct ordering in
+  file listings. A file is not created if the corresponding stream is empty.
+  Any intermediate directories in the resulting output files will automatically
+  be created if necessary. If ``filename`` is a relative path, it will be
+  converted to an absolute path based on the directory where :ref:`mpirun(1)
+  <man1-mpirun>` is executed. Note that this will not work in environments
+  where the file system on compute nodes differs from that where
+  :ref:`mpirun(1) <man1-mpirun>` is executed.
 
 * ``--stdin <rank>``: The MPI_COMM_WORLD rank of the process that is
   to receive stdin.  The default is to forward stdin to MPI_COMM_WORLD