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.

library/alloc/src/vec/mod.rs

@@ -2167,6 +2167,12 @@ impl<T, A: Allocator> Vec<T, A> {
     /// `[at, len)`. After the call, the original vector will be left containing
     /// the elements `[0, at)` with its previous capacity unchanged.
     ///
+    /// - If you want to take ownership of the entire contents and capacity of
+    ///   the vector, see [`mem::take`] or [`mem::replace`].
+    /// - If you don't need the returned vector at all, see [`Vec::truncate`].
+    /// - If you want to take ownership of an arbitrary subslice, or you don't
+    ///   necessarily want to store the removed items in a vector, see [`Vec::drain`].
+    ///
     /// # Panics
     ///
     /// Panics if `at > len`.