stm32/timer/simplepwm: Fix docs formatting and clarify timer usage

[plaes]

Firstly, fix code formatting inside docstring and then hopefully clarify the timer availability a bit more.

[lulf]

You should add the ignore attribute as well.

[plaes]

You should add the ignore attribute as well.

Thanks, added. Also updated the same note for waveform_up docstring.

Would it make sense to add a method that would automatically infer correct DMAx_CHy based on the used timer/channel? For example currently we do:

let mut foo = SimplePwm::new(d.TIM4, ...);
foo.waveform_up(dp.DMA1_CH7.reborrow(), pwm_channel, color).await;

Or can't we somehow infer the DMA instance and channel based on the timer and fill in the DMA1_CH7 automatically?