RawSerial vs Serial - document the known limitations

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Firstly, we should document better Serial and RawSerial implementation. Serial has known limitation which were identified by community for instance on mbed.org or are know by developers. But it's not stated in the documentation (RawSerial seems to have copy-paste text from Serial class at first sight).

Some references: https://developer.mbed.org/questions/54454/Best-solution-for-serial-input-SerialBas/
Test program to compare RawSerial/Serial for instance, https://developer.mbed.org/users/sam_grove/code/data_loss_test/file/73617eecf751/main.cpp