Clean up English and clarify a point regarding process binding

in the mpirun man page.

This commit was SVN r23558.

orte/tools/orterun/orterun.1in

@@ -865,7 +865,7 @@ turned off and no bindings are reported.
 .
 .PP
 Open MPI's support for process binding depends on the underlying
-operating system.  Therefore, processing binding may not be available
+operating system.  Therefore, certain process binding options may not be available
 on every system.
 .
 .PP