Deprecate `:return-altered-zipper` and write documentation for `:new-zloc` approach.

[kkinnear]

The original way to modify the actual code was to use :return-altered-zipper in the options map, but that has been replaced by a new approach which is more widely applicable where any option-fn can return a "new" structure (or actual bloc) in :new-zloc. Both of the deprecation of :return-altered-zipper and reasonable documentation of :new-zloc seem to have been missed, and need to be added to the docs.