Documentation of `hana::apply` should link to `hana::unpack`

I have read the rationale behind the naming (and parameter order) of `unpack`. I agree with the parameter order, and I have no strong feelings about the name. However, whenever I need `unpack`, I cannot recall its name, only finding `apply` in my memory. I suspect that there are other people around who are more accustomed to `apply` than `unpack`. Maybe a cross-link in the documentation could help here?

Read the documentation for hana::apply and hana::unpack first. Add a cross-link from the hana::apply documentation to hana::unpack, then verify that the link is present and points to the correct documentation entry.

Written by the indexing model from the issue text.