Help: Add a hint about C and C++ compiler version variable

Closes #16917

Help/variable/CMAKE_LANG_COMPILER_VERSION.rst

@@ -6,3 +6,7 @@ Compiler version string.
 Compiler version in major[.minor[.patch[.tweak]]] format.  This
 variable is not guaranteed to be defined for all compilers or
 languages.
+
+For example ``CMAKE_C_COMPILER_VERSION`` and
+``CMAKE_CXX_COMPILER_VERSION`` might indicate the respective C and C++
+compiler version.