doc: Edit the PWM driver APIs

Edit public API details for clarity, grammar, punctuation found
on file pwm.h

Made consistent use of @retval to document return values.

Used imperative verbs for all @brief descriptions.

Change-Id: I5a0f6290361e499ceb911473338358128f85e25f
Signed-off-by: Rodrigo Caballero <rodrigo.caballero.abraham@intel.com>
1 file changed