[ruby/date] Describe what is meant by valid in the Date.valid_date? rdoc

https://github.com/ruby/date/commit/8eca79d1f0

ext/date/date_core.c

@@ -2545,9 +2545,12 @@ date_s__valid_civil_p(int argc, VALUE *argv, VALUE klass)
  *    Date.valid_date?(year, month, mday[, start=Date::ITALY])   ->  bool
  *
  * Returns true if the given calendar date is valid, and false if not.
+ * Valid in this context is whether the arguments passed to this
+ * method would be accepted by ::new.
  *
  *    Date.valid_date?(2001,2,3)	#=> true
  *    Date.valid_date?(2001,2,29)	#=> false
+ *    Date.valid_date?(2001,2,-1)	#=> true
  *
  * See also ::jd and ::civil.
  */