Improve slicing/dicing documentation.

doc/mercury_user_guide.texi:
    Clarify the documentation of slices and dices, partly by
    improving exposition, partly by including previously ignored info.

    Link two near-duplicate blocks of text to each other.

mdbcomp/slice_and_dice.m:
    Replace stringly typed code with strongly typed code:
    parsing instead of validating.

    Clarify comments.

slice/mslice.m:
    Conform to the change in slice_and_dice.m.