PM / Runtime: Correct documentation of pm_runtime_irq_safe()

The description of pm_runtime_irq_safe() has to be updated to follow
the code after commit 02b26774

 (PM / Runtime: Allow _put_sync() from
interrupts-disabled context).

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Acked-by: Kevin Hilman <khilman@ti.com>