Add documentation to explain which parts of VPR are parellel

Author: AmirhosseinPoolad

doc/src/vpr/command_line_usage.rst

@@ -223,6 +223,11 @@ General Options
 
     If this option is not specified it may be set from the ``VPR_NUM_WORKERS`` environment variable; otherwise the default is used.
 
+    If this option is set to something other than 1, the following algorithms can be run in parallel:
+    *  Timing Analysis
+    *  Routing (If routing algorithm is set to parallel or parallel_decomp; See :option:`--router_algorithm`)
+    *  Portions of analytical placement (Is using the analytical placement flow and compile VPR with Eigen; See :option:`--analytical_place`)
+
     .. note:: To compile VPR to allow the usage of parallel workers, ``libtbb-dev`` must be installed in the system.
 
     **Default:** ``1``