gc.c: add note about experimental nature of new GC.start flags

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44028 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

gc.c

@@ -5002,6 +5002,10 @@ Init_stack(volatile VALUE *addr)
  *
  *  Use full_mark: false to perform a minor GC.
  *  Use immediate_sweep: false to defer sweeping (use lazy sweep).
+ *
+ *  Note: These keyword arguments are implementation and version dependent. They
+ *  are not guaranteed to be future-compatible, and may be ignored if the
+ *  underlying implementation does not support them.
  */
 
 static VALUE