Add comment to Abort match arm

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

compiler/rustc_mir/src/transform/check_consts/validation.rs

@@ -882,6 +882,7 @@ impl Visitor<'tcx> for Validator<'mir, 'tcx> {
             }
 
             TerminatorKind::Abort => {
+                // Cleanup blocks are skipped for const checking (see `visit_basic_block_data`).
                 span_bug!(self.span, "`Abort` terminator outside of cleanup block")
             }