Add documentation for return types and pointer types in witx.

[sunfishcode]

Document how return types, in particular expected return types, work in witx.

Also update the witx-docs.md file to describe the current status of witx and the transition to wit.

[sunfishcode]

I've now also added documentation for pointer types, including alignment behavior.

[sunfishcode]

Sounds good; let's merge this, and if anyone has further questions, we can iterate in new issues and PRs.