Add backwards compatibility example macro for rb_eval_cmd_kwd [ci skip]

doc/extension.rdoc

@@ -1946,6 +1946,7 @@ the <code>*_kw</code> functions introduced in Ruby 2.7.
   #define rb_proc_call_with_block_kw(p, c, v, b, kw) rb_proc_call_with_block(p, c, v, b)
   #define rb_method_call_kw(c, v, m, kw) rb_method_call(c, v, m)
   #define rb_method_call_with_block_kw(c, v, m, b, kw) rb_method_call_with_block(c, v, m, b)
+  #define rb_eval_cmd_kwd(c, a, kw) rb_eval_cmd(c, a, 0)
   #endif
 
 == Appendix C. Functions available for use in extconf.rb