grammar: reorder and add comment

Author: hauntsaninja

Grammar/python.gram

@@ -587,12 +587,14 @@ del_target[expr_ty] (memo):
     | a=t_primary '.' b=NAME !t_lookahead { _Py_Attribute(a, b->v.Name.id, Del, EXTRA) }
     | a=t_primary '[' b=slices ']' !t_lookahead { _Py_Subscript(a, b, Del, EXTRA) }
     | del_t_atom
-    | invalid_del_target
 del_t_atom[expr_ty]:
+# The lookahead here ensures that we don't match expressions where a prefix of the expression is
+# a name (like `del a + b`), thereby letting these cases fall through to invalid_del_target.
     | a=NAME &(')' | ']' | ',' | ';' | NEWLINE) { _PyPegen_set_expr_context(p, a, Del) }
     | '(' a=del_target ')' { _PyPegen_set_expr_context(p, a, Del) }
     | '(' a=[del_targets] ')' { _Py_Tuple(a, Del, EXTRA) }
     | '[' a=[del_targets] ']' { _Py_List(a, Del, EXTRA) }
+    | invalid_del_target
 
 targets[asdl_seq*]: a=','.target+ [','] { a }
 target[expr_ty] (memo):