PM / Sleep: Update freezer documentation

This patch:
 * Substitutes some obsolete references to kernel/power/process.c by
   kernel/freezer.c.
 * Mentions kernel/freezer.c as being part of the "freezer" code along
   with the rest of the files.
 * Fixes a trivial typo.

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