Rollup merge of #117039 - scottmcm:clarify-get-unchecked, r=cuviper

Clarify UB in `get_unchecked(_mut)`

Inspired by #116915, it was unclear to me what exactly "out-of-bounds index" means in `get_unchecked`.

One could [potentially](https://rust.godbolt.org/z/hxM764orW) interpret it that `get_unchecked` is just another way to write `offset`, but I think `get_unchecked(len)` is supposed to be UB even though `.offet(len)` is well-defined (as is `.get_unchecked(..len)`), so write that more directly in the docs.

**libs-api folks**: Can you confirm whether this is what you expect this to mean?  And is the situation any different for `<*const [T]>::get_unchecked`?

library/core/src/slice/mod.rs

@@ -640,6 +640,11 @@ impl<T> [T] {
     /// Calling this method with an out-of-bounds index is *[undefined behavior]*
     /// even if the resulting reference is not used.
     ///
+    /// You can think of this like `.get(index).unwrap_unchecked()`.  It's UB
+    /// to call `.get_unchecked(len)`, even if you immediately convert to a
+    /// pointer.  And it's UB to call `.get_unchecked(..len + 1)`,
+    /// `.get_unchecked(..=len)`, or similar.
+    ///
     /// [`get`]: slice::get
     /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
     ///
@@ -675,6 +680,11 @@ impl<T> [T] {
     /// Calling this method with an out-of-bounds index is *[undefined behavior]*
     /// even if the resulting reference is not used.
     ///
+    /// You can think of this like `.get_mut(index).unwrap_unchecked()`.  It's
+    /// UB to call `.get_unchecked_mut(len)`, even if you immediately convert
+    /// to a pointer.  And it's UB to call `.get_unchecked_mut(..len + 1)`,
+    /// `.get_unchecked_mut(..=len)`, or similar.
+    ///
     /// [`get_mut`]: slice::get_mut
     /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
     ///