Help: Move linker preference variables to the internal section

Merged Craig Scott requested to merge craig.scott/cmake:doc-internal-linker-preference into master

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.

See the forum thread where this was discussed:

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