Help: Move linker preference variables to the internal section

These probably should not have been documented as public variables
to begin with. But since they have been documented for a long time,
we can't just remove them from the docs. Move them to the internal
section instead to make it clearer that they are not intended to be used
directly by projects.

Help/manual/cmake-variables.7.rst

@@ -581,8 +581,6 @@ Variables for Languages
    /variable/CMAKE_LANG_IMPLICIT_LINK_LIBRARIES
    /variable/CMAKE_LANG_LIBRARY_ARCHITECTURE
    /variable/CMAKE_LANG_LINK_EXECUTABLE
-   /variable/CMAKE_LANG_LINKER_PREFERENCE
-   /variable/CMAKE_LANG_LINKER_PREFERENCE_PROPAGATES
    /variable/CMAKE_LANG_LINKER_WRAPPER_FLAG
    /variable/CMAKE_LANG_LINKER_WRAPPER_FLAG_SEP
    /variable/CMAKE_LANG_OUTPUT_EXTENSION
@@ -727,6 +725,8 @@ are subject to change, and not recommended for use in project code.
    /variable/CMAKE_LANG_COMPILER_ABI
    /variable/CMAKE_LANG_COMPILER_ARCHITECTURE_ID
    /variable/CMAKE_LANG_COMPILER_VERSION_INTERNAL
+   /variable/CMAKE_LANG_LINKER_PREFERENCE
+   /variable/CMAKE_LANG_LINKER_PREFERENCE_PROPAGATES
    /variable/CMAKE_LANG_PLATFORM_ID
    /variable/CMAKE_NOT_USING_CONFIG_FLAGS
    /variable/CMAKE_VS_INTEL_Fortran_PROJECT_VERSION

Help/variable/CMAKE_LANG_LINKER_PREFERENCE.rst

@@ -1,6 +1,8 @@
 CMAKE_<LANG>_LINKER_PREFERENCE
 ------------------------------
 
+An internal variable subject to change.
+
 Preference value for linker language selection.
 
 The "linker language" for executable, shared library, and module

Help/variable/CMAKE_LANG_LINKER_PREFERENCE_PROPAGATES.rst

@@ -1,6 +1,8 @@
 CMAKE_<LANG>_LINKER_PREFERENCE_PROPAGATES
 -----------------------------------------
 
+An internal variable subject to change.
+
 True if :variable:`CMAKE_<LANG>_LINKER_PREFERENCE` propagates across targets.
 
 This is used when CMake selects a linker language for a target.