Docs: mention the requirement to discoverService() before reading a characteristic

[nevyn]

At: https://www.arduino.cc/reference/en/libraries/arduinoble/bledevice.characteristic/

It would be helpful to mention in the main documentation body for BLEDevice.characteristic() that the API requires you to either BLEDevice.discoverAttributes() or BLEDevice.discoverSerivce() first. It is hinted at from the example, but it's really easy to miss.

It could also be mentioned in the conceptual documentation over at https://www.arduino.cc/reference/en/libraries/arduinoble/ that characteristics need to be discovered before being queried. There are sections about notifying and writing characteristics, but no section about basic reading of characteristics; that could be a good place to mention it.

Thanks!