document the need of t24p calibration
Currently the documentation of WRPC doesn't mention that even if WRPC is configured to run in Master/GrandMaster mode, it has to be run at least once in Slave mode. That is because automatic t24 phase transition point calibration procedure is called automatically in Slave mode, the result of measurement is stored to EEPROM and can be later used in Master/GrandMaster mode. Forgetting that may result in generating wrong timestamps once in a while, which causes jumps in RTT and PTP Offset.