clarify provenance issue in new_owned

src/lib.rs

@@ -94,6 +94,9 @@ impl Stringish {
     /// Alternatively, use the [`Ish::ish`] method.
     #[must_use]
     pub fn new_owned(s: String) -> Self {
+        // converting to a `Vec` is necessary since `s.as_mut_ptr()` would call `str::as_mut_ptr`
+        // through deref coercion, which would only give us a pointer with provenance to the
+        // initialized part of the `String`, which is not enough to deallocate.
         let mut v = ManuallyDrop::new(s.into_bytes());
         Self {
             // SAFETY: `v.as_mut_ptr()` is never null