Added docstrings to RandomMteTag struct

cranelift/wasm/src/state.rs

@@ -282,8 +282,11 @@ static ALL_POSSIBLE_RANDOM_MTE_TAGS: Lazy<(usize, Vec<u8>)> = Lazy::new(|| {
 /// from `EXCLUDED_MTE_TAGS`.
 #[derive(Debug)]
 struct RandomMteTag {
+    /// The 4-bit random MTE tag.
     tag: u8,
-    // TODO: we could probably make this a u8 for efficieny as well, but vector.len() natively returns usize, so we use that for now
+    /// The index that was used to index into `ALL_POSSIBLE_RANDOM_MTE_TAGS` to
+    /// receive `tag`. This is stored here because it is needed to generate the
+    /// next random tag.
     index: usize,
 }