Documentation: driver-api: pps: Update PPS generator documentation

PPS documentation has a generalized section for generators. Update the
section so any new generator documentation can be appended.

Signed-off-by: Pandith N <pandith.n@intel.com>
Signed-off-by: Lakshmi Sowjanya D <lakshmi.sowjanya.d@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Rodolfo Giometti <giometti@enneenne.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20231016101953.27308-1-lakshmi.sowjanya.d@intel.com