Update src/stacked_borrows.rs

Co-authored-by: Ralf Jung <[email protected]>

Author: saethlin

src/stacked_borrows.rs

@@ -102,7 +102,8 @@ pub struct GlobalStateInner {
     base_ptr_tags: FxHashMap<AllocId, SbTag>,
     /// Next unused call ID (for protectors).
     next_call_id: CallId,
-    /// All tags currently protected
+    /// All currently protected tags.
+    /// An item is protected if its tag is in this set, *and* it has the "protected" bit set.
     /// We add tags to this when they are created with a protector in `reborrow`, and
     /// we remove tags from this when the call which is protecting them returns, in
     /// `GlobalStateInner::end_call`. See `Stack::item_popped` for more details.