Revert accidental comment deletion

compiler/rustc_borrowck/src/lib.rs

@@ -821,6 +821,8 @@ fn visit_terminator_after_primary_effect(
             | TerminatorKind::Return
             | TerminatorKind::TailCall { .. }
             | TerminatorKind::CoroutineDrop => {
+                // Returning from the function implicitly kills storage for all locals and statics.
+                // Often, the storage will already have been killed by an explicit
                 // StorageDead, but we don't always emit those (notably on unwind paths),
                 // so this "extra check" serves as a kind of backup.
                 let borrow_set = self.borrow_set.clone();