In the slices.zig doctest, the code
`const all_together_slice = all_together[0..]` is incorrect, since the
actual type is a pointer to an array, instead of a slice.

Use a runtime-known value to slice the array.

In the next "slice pointer" test, clarify that slicing a slice to
produce a pointer to an array, requires comptime-known indexes, not just
constant indexes.