Improve how assistant trigger types are explained

[sds100]

Developer TODO (don't remove)

• update documentation
• Explain when creating an assistant trigger what all the options mean. The secondary app is used when you want to remap the "Side key" on Samsung devices. "Device assistant" is useful on pixels for long pressing the power button. Voice assistant helps with remapping assistant buttons on headsets and keyboards.
• Rename the options to "side key/power button" and "voice assistant".

See #1433.

[sds100]

implemented