pwm: pxa: Add reference manual link and limitations

Add a pointer to the location of reference manuals for some of the
supported chips, and add a limitations section explaining the hardware's
PWM disable behavior.
Suggested-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Doug Brown <doug@schmorgal.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20221113233639.24244-6-doug@schmorgal.comSigned-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

drivers/pwm/pwm-pxa.c

@@ -6,6 +6,13 @@
  *
  * 2008-02-13	initial version
  *		eric miao <eric.miao@marvell.com>
+ *
+ * Links to reference manuals for some of the supported PWM chips can be found
+ * in Documentation/arm/marvell.rst.
+ *
+ * Limitations:
+ * - When PWM is stopped, the current PWM period stops abruptly at the next
+ *   input clock (PWMCR_SD is set) and the output is driven to inactive.
  */
 
 #include <linux/module.h>