Improve documentation for Vec::as_slice

[mbrubeck]

Vec::as_slice and Vec::as_mut_slice are both marked unstable with reason "waiting on RFC revision." But the docs for as_mut_slice say that it is deprecated.

If these methods are deprecated, they should be marked deprecated instead of unstable. And either way, a hint should be added regarding how to replace Vec::as_slice in old code.

[moved from rust-lang/rfcs#1128]

[mbrubeck]

cc @bluss

[steveklabnik]

See #25833 (comment) , this is actually correct

[mbrubeck]

There are still these problems to be fixed:

• as_mut_slice docs say it is deprecated, when it isn't.
• as_slice docs don't point to the stable alternative &v[..].

[steveklabnik]

as_mut_slice docs say it is deprecated, when it isn't.

Which are these? It shows as unstable in that patch I worked up

[steveklabnik]

ahhh i see, it's in the words, not the attribute