Mention SliceConcatExt's stability in its docs

SliceConcatExt's status as an unstable trait with stable methods is
documented in the compiler error for using it, and in
https://doc.rust-lang.org/std/prelude/, but it is not mentioned in the
trait itself.

Mentioning the methods can be used in stable rust today should help
users who are looking for a `join` method while working on stable rust.

src/liballoc/slice.rs

@@ -1725,6 +1725,14 @@ impl [u8] {
            reason = "trait should not have to exist",
            issue = "27747")]
 /// An extension trait for concatenating slices
+///
+/// While this trait is unstable, the methods are stable. `SliceConcatExt` is
+/// included in the [standard library prelude], so you can use [`join()`] and
+/// [`concat()`] as if they existed on `[T]` itself.
+///
+/// [standard library prelude]: ../../std/prelude/index.html
+/// [`join()`]: #tymethod.join
+/// [`concat()`]: #tymethod.concat
 pub trait SliceConcatExt<T: ?Sized> {
     #[unstable(feature = "slice_concat_ext",
                reason = "trait should not have to exist",