Update PWM driver page to inform user of the different options for when PWM duty ratios are updated

[elsevers]

Abstract

Update PWM driver page to inform user of the different options for when PWM duty ratios are updated

Context

Since AMDC v1.3, the PWM interface has several options for when it outputs duty ratios:

Our docs.amdc.dev page on this interface is very brief and does not cover this.

https://github.com/Severson-Group/AMDC-Firmware/blob/18321d8316e14fc71e97e367b17600adbe26f67b/sdk/app_cpu1/common/drv/pwm.h#L63-L67

Approach

Add a short section to the docs.amdc.dev page on the PWM interface that explains the options for when duty ratios are updated.