add high-level context of explanation for server mode into command_line_usage.rst

Author: w0lek

doc/src/vpr/command_line_usage.rst

@@ -1826,6 +1826,9 @@ The following options are used to enable power estimation in VPR.
 
 Server Mode Options
 ^^^^^^^^^^^^^^^^^^^^^^^^
+
+If VPR is in server mode, it listens on a socket for commands from a client. Currently, this is used to enable interactive timing analysis and visualization of timing paths in the VPR UI under the control of a separate client.
+
 The following options are used to enable server mode in VPR.
 
 .. seealso:: :ref:`server_mode` for more details.