PM / Sleep: Recommend [un]lock_system_sleep() over using pm_mutex directly

Update the documentation to explain the perils of directly using
mutex_[un]lock(&pm_mutex) and recommend the usage of the safe
APIs [un]lock_system_sleep() instead.

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