Clarify kpropd standalone mode documentation

The kpropd -S option is no longer needed to run kpropd in standalone
mode, but its functionality is not deprecated; standalone mode is
automatically activated when appropriate.  Clarify the kpropd
documentation on standalone mode to avoid giving the impression that
the mode is deprecated.

ticket: 7751 (new)
target_version: 1.12
tags: pullup