[ADC][PWM] Improve enum name and better document Handshake for synchronous measurment.

[jalinei]

Following hackathon feedback :
It seems still hard to understand how to configure the external event that links ADC and PWM

Maybe a slight rework is necessary in the enum to change the naming and make it dumb proof ?

• hrtim_ev1 and ADCTRIG_1 are not self explanatory.
• hrtim is not self explanatory we should use MEAS_TRIG_1 or something like that
• Both enum should be indentical on both sides.

In the documentation we need a dedicated paragraph to explain this handshake as it is not trivial.

[cfoucher-laas]

In addition, changing event source in Data API should also automatically configure it on the PWM side.