RawSerial vs Serial - document the known limitations

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

Start by comparing the Serial and RawSerial class documentation, then review the linked main.cpp data_loss_test and community reference. Record the confirmed limitations and differences in the relevant documentation, with completion shown by both classes having accurate, non-duplicated guidance.

Written by the indexing model from the issue text.