Update src/machine.rs

Co-authored-by: Ralf Jung <post@ralfj.de>

Author: saethlin

src/machine.rs

@@ -61,6 +61,9 @@ pub struct FrameData<'tcx> {
     /// `stacked_borrows::GlobalState` upon function return, and if we attempt to pop a protected
     /// tag, to identify which call is responsible for protecting the tag.
     /// See `Stack::item_popped` for more explanation.
+    ///
+    /// This will contain one tag per reference passed to the function, so
+    /// a size of 2 is enough for the vast majority of functions.
     pub protected_tags: SmallVec<[SbTag; 2]>,
 }