Doc: Clarify how to use ++runscript (#3581)

Co-authored-by: Matthias Diener <mdiener@illinois.edu>

doc/charm++/manual.rst

@@ -12186,15 +12186,19 @@ The remaining options cover details of process launch and connectivity:
    Print help messages
 
 ``++runscript``
-   Script to run node-program with. The specified run script is invoked
-   with the node program and parameter. For example:
+   Script to run program with. The specified run script is invoked with the
+   given program and arguments as parameters. This is useful for loading modules
+   or setting up an environment prior the running the program. For example:
 
    .. code-block:: bash
 
       $ ./charmrun +p4 ./pgm 100 2 3 ++runscript ./set_env_script
 
-   In this case, the ``set_env_script`` is invoked on each node before
-   launching ``pgm``.
+   In this case, ``set_env_script`` is invoked on each node. **Note:** When this
+   is provided, ``charmrun`` will not invoke the program directly, instead only
+   invoking the script with the program name and program arguments passed in as
+   script parameters; thus, ``set_env_script`` should generally end with ``$*``
+   to actually start the program.
 
 ``++xterm``
    Which xterm to use