pwm: Document that the pinstate of a disabled PWM isn't reliable

Some (most?) drivers emit the inactive state when the PWM is disabled.
However there are exceptions, so document that a consumer better doesn't
depend on this behaviour.

Some known exceptions:

  - imx27 emits 0 independent of the configured polarity
  - mxs just drives the output to the last emitted state.
  - iqs620a makes the output tristated on disable, so an external
    pull-down would be required.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>