Help: Document QCC and SCO CMAKE_<LANG>_COMPILER_ID

- QNX C/C++ compiler is identified in `CMAKE_<LANG>_COMPILER_ID` as
  value "QCC". The qcc/q++ are frontend compiler commands that invoke
  the GNU compiler (gcc/g++) as a backend compiler, but it's treated as
  a separate compiler with its own ID for better determination.

- The C/C++ compiler on UnixWare/OpenServer is identified in
  `CMAKE_<LANG>_COMPILER_ID` as value "SCO", (compiler that defines the
  `__SCO_VERSION__` preprocessor macro).