Enhanced documentation for linkCheck()

[avbentem]

Improved the documentation to mitigate confusion that showed in https://www.thethingsnetwork.org/forum/t/ttn-node-link-check-does-not-work/19981/18

• Calling linkCheck before joining has no effect.

• The linkCheck command is delegated to the RN2483 module by using mac set linkchk, which is documented as:

  When the time interval expires, the next application packet that will be sent to the server will include also a link check MAC command.

[johanstokking]

Released in https://github.com/TheThingsNetwork/arduino-device-lib/releases/tag/v2.5.14

Thanks @avbentem and @jpmeijers