PM / Freezer / Docs: Document the beauty of freeze/thaw semantics

PM / Freezer / Docs: Document the beauty of freeze/thaw semantics

The way the different freeze/thaw functions encapsulate each other are quite
lovely from a design point of view. And as a side-effect, the way in which
they are invoked (cleaning up on failure for example) differs significantly
from how usual functions are dealt with. This is because of the underlying
semantics that govern the freezing and thawing of various tasks.

This subtle aspect that differentiates these functions from the rest, is
worth documenting.

Many thanks to Tejun Heo for providing enlightenment on this topic.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>