Clarify a comment in RegionConstraintCollector::leak_check.

Author: nnethercote

src/librustc/infer/region_constraints/leak_check.rs

@@ -14,9 +14,11 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
     /// retain the older (arguably incorrect) behavior of the
     /// compiler.
     ///
-    /// NB. The use of snapshot here is mostly an efficiency thing --
-    /// we could search *all* region constraints, but that'd be a
-    /// bigger set and the data structures are not setup for that. If
+    /// NB. Although `_snapshot` isn't used, it's passed in to prove
+    /// that we are in a snapshot, which guarantees that we can just
+    /// search the "undo log" for edges. This is mostly an efficiency
+    /// thing -- we could search *all* region constraints, but that'd be
+    /// a bigger set and the data structures are not setup for that. If
     /// we wind up keeping some form of this check long term, it would
     /// probably be better to remove the snapshot parameter and to
     /// refactor the constraint set.