GH-92678: Document that you shouldn't be doing your own dictionary offset calculations. (GH-95598) (GH-95821)

Co-authored-by: Petr Viktorin <encukou@gmail.com>
Co-authored-by: Stanley <46876382+slateny@users.noreply.github.com>
Co-authored-by: Mark Shannon <mark@hotpy.org>