put back the n_procs docs

Author: chrisgorgo

doc/users/plugins.rst

@@ -78,6 +78,11 @@ To distribute processing on a multicore machine, simply call::
 
   workflow.run(plugin='MultiProc')
 
+This will use all available CPUs. If on the other hand you would like to restrict
+the number of used resources (to say 2 CPUs), you can call::
+
+  workflow.run(plugin='MultiProc', plugin_args={'n_procs' : 2}
+
 IPython
 -------