Note about Regexp.linera_time? [ci skip]

re.c

@@ -4204,6 +4204,16 @@ rb_reg_s_union_m(VALUE self, VALUE args)
  *
  *    Regexp.linear_time?(/re/) # => true
  *
+ *  Note that this is a property of the ruby interpreter, not of the argument
+ *  regular expression.  Identical regexp can or cannot run in linear time
+ *  depending on your ruby binary.  Neither forward nor backward compatibility
+ *  is guaranteed about the return value of this method.  Our current algorithm
+ *  is (*1) but this is subject to change in the future.  Alternative
+ *  implementations can also behave differently.  They might always return
+ *  false of everything.
+ *
+ *  (*1): https://doi.org/10.1109/SP40001.2021.00032
+ *
  */
 static VALUE
 rb_reg_s_linear_time_p(int argc, VALUE *argv, VALUE self)