error.c: fixed doc [ci skip]

* error.c (exc_message): fixed doc regarding the presence of
  to_str.  [Fix GH-1517]

Author:    Marc Gauthier <marcg.gauthier@gmail.com>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57405 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

error.c

@@ -894,9 +894,7 @@ exc_to_s(VALUE exc)
  *   exception.message   ->  string
  *
  * Returns the result of invoking <code>exception.to_s</code>.
- * Normally this returns the exception's message or name. By
- * supplying a to_str method, exceptions are agreeing to
- * be used where Strings are expected.
+ * Normally this returns the exception's message or name.
  */
 
 static VALUE