(DOC-1035) Notes for enabling profiling output

Added some additional information to the notes on enabling profile output for
Puppet Masters. Notably:

  - Log destination and level _absolutely must_ be specified as command line
    arguments and not setting in `puppet.conf`. This requires editing
    `config.ru` when Puppet is running under Passenger.

  - Added a pointer to the PE `config.ru` file used by the Puppet Master.

  - Added a note concerning `syslog` priority levels. Some systems, such as
    Enterprise Linux, don't record messages sent to syslog at debug priority
    unless the configuration is changed.