In d2efc905 (Help: Move linker preference variables to the internal
section, 2022-01-15), the internal nature of the
CMAKE_<LANG>_LINKER_PREFERENCE and
CMAKE_<LANG>_LINKER_PREFERENCE_PROPAGATES variables was
made clearer. The LINKER_LANGUAGE target property references
the former, but doesn't mention that the variable is internal. Make
the internal nature of the variable and the preference implementation
in general clearer in the LINKER_LANGUAGE docs.