doc: services: pm: Reword System PM Doc

The following changes:
- Generally reword all the paragraphs to hopefully by clearer and
  easier to read by using more straighforward and direct wording,
  and avoiding run on sentences and ambiguous/wrong grammar, etc.
- Move Examples to the bottom of the page.
- Fix a typo in the code expression where there was an extra
  parentheses.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>