pwm: Adapt sysfs API documentation to reality

Most low-level PWM drivers support duty_cycle == period, and so does the
sysfs API. Also polarity can be changed for enabled PWMs since commit
39100ce ("pwm: Switch to the atomic API").

Reported-by: Jens Gehrlein <J.Gehrlein@eckelmann.de>
Link: https://lore.kernel.org/r/20230911154454.675057-1-u.kleine-koenig@pengutronix.de
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>

Author: Uwe Kleine-König

Documentation/driver-api/pwm.rst

@@ -111,13 +111,13 @@ channel that was exported. The following properties will then be available:
 
   duty_cycle
     The active time of the PWM signal (read/write).
-    Value is in nanoseconds and must be less than the period.
+    Value is in nanoseconds and must be less than or equal to the period.
 
   polarity
     Changes the polarity of the PWM signal (read/write).
     Writes to this property only work if the PWM chip supports changing
-    the polarity. The polarity can only be changed if the PWM is not
-    enabled. Value is the string "normal" or "inversed".
+    the polarity.
+    Value is the string "normal" or "inversed".
 
   enable
     Enable/disable the PWM signal (read/write).