add note about MAX_ALLOC_LIMIT

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

Author: syvb

Diff for: compiler/rustc_mir/src/transform/const_prop.rs

@@ -181,7 +181,7 @@ impl<'mir, 'tcx> ConstPropMachine<'mir, 'tcx> {
 
 impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for ConstPropMachine<'mir, 'tcx> {
     compile_time_machine!(<'mir, 'tcx>);
-    const PANIC_ON_ALLOC_FAIL: bool = true; // all allocations are small
+    const PANIC_ON_ALLOC_FAIL: bool = true; // all allocations are small (see `MAX_ALLOC_LIMIT`)
 
     type MemoryKind = !;