PM / Freezer / Docs: Update documentation about freezing of tasks

PM / Freezer / Docs: Update documentation about freezing of tasks

The file Documentation/power/freezing-of-tasks.txt was still referencing
the TIF_FREEZE flag, that was removed by the commit
0e2c10603262124383b1f187413c1bfa2224892a(freezer: remove now unused
TIF_FREEZE).

This patch removes all the references of TIF_FREEZE that were left
behind.

Signed-off-by: Marcos Paulo de Souza <marcos.souza.org@gmail.com>
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>