PM / OPP: improve introductory documentation

Make Operating Performance Points (OPP) library introductory chapter
a little more reader-friendly. Split the chapter into two sections,
highlight the definition with an example and minor rewording to be
verbose.

Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>