docs: pm: Replace outdated documentation

Power management documentation was still using "deep sleep" and
"sleep" concepts. This commit just remove them, since they are not
longer used, and introduce the new power states defined in pm_state.

As each state has its own meaning and it is documented in the API,
lets just reference them here and avoid getting out of sync.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>