Rollup merge of #120180 - Zalathar:vec-split-off-alternatives, r=dtolnay

Document some alternatives to `Vec::split_off`

One of the discussion points that came up in #119917 is that some people use `Vec::split_off` in cases where they probably shouldn't, because the alternatives (like `mem::take`) are hard to discover.

This PR adds some suggestions to the documentation of `split_off` that should point people towards alternatives that might be more appropriate for their use-case.

I've deliberately tried to keep these changes as simple and uncontroversial as possible, so that they don't depend on how the team decides to handle the concerns raised in #119917. That's why I haven't touched the existing documentation for `split_off`, and haven't added links to `split_off` to the documentation of other methods.

Author: matthiaskrgr