Help: Highlight internal nature of linker preference variables
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.
This cleans up a minor point highlighted in !6871 (comment 1112202)
Edited by Craig Scott