A round of tidying up the documentation,
worth a patch version bump.
This also adds a checklist to `Vec`
to document the state of API support.
It would be good to clarify
which version of the standard `Vec` and `slice` APIs
this code is wrapping,
and also to add some machine-checking
that the docs accurately reflect the real API support.
As it stands, these can all very easily get out of sync.