Document "var_monitor -c" for clean up

[tpatki]

Users are not using the cleanup functionality and this is causing data files not being generated. We need to add this to the documentation for var_monitor.
Thanks @francescoantici for pointing this out.

[tpatki]

Document that only one instance per node is needed for var_monitor (as we don't support multi-tenancy), and running multiple instances of var_monitor will create only one file on the node, and others will return null for their output files (not overwrite the one file).

Thanks @kulnaman and @hariharan-devarajan for testing this.