Document Kenji's multidimensional array support implementation.
Merge with existing docs on opIndex / opSlice / opDollar.
Update opSliceUnary docs.

Update opSliceOpAssign docs.
Base array object is only evaluated once even in complex expressions.
opDollar!i is also only evaluated once per dimension.
Merge pull request #625 from quickfur/multidim_overload

Document multidimensional array op overloading