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).

Help/variable/CMAKE_LANG_COMPILER_ID.rst

@@ -41,6 +41,8 @@ Value                           Name
 ``OpenWatcom``                  `Open Watcom`_
 ``PGI``                         The Portland Group
 ``PathScale``                   PathScale
+``QCC``                         QNX C/C++ compiler
+``SCO``                         SCO OpenServer/UnixWare C/C++ Compiler
 ``SDCC``                        `Small Device C Compiler`_
 ``SunPro``                      Oracle Developer Studio
 ``Tasking``                     `Tasking Compiler Toolsets`_