gpio: document interaction between WaitForEdge and Halt

[maruel]

This issue is two part:

• https://periph.io/x/periph/conn/gpio doesn't document that Halt is the formal way to unblock a WaitForEdge function.
• https://periph.io/device/button/ and https://periph.io/device/pir/ should demonstrate that in a copy-pastable way.