The return value of rb_class2name and rb_obj_classname

[kubo]

In MRI, rb_class2name and rb_obj_classname are declared as follows:

const char *rb_class2name(VALUE);
const char *rb_obj_classname(VALUE);

The return value must not be freed.

In Rubinius, they are declared as follows:

  /** C string representation of the class' name. You must free this string. */
  char*   rb_class2name(VALUE class_handle);

  /** String representation of the object's class' name. You must free this string. */
  char*   rb_obj_classname(VALUE object_handle);

They are noticed that "You must free this string." But it must not be freed as far as I checked.
The notice should be deleted and the return type should be changed to 'const char *' as MRI does.

[evanphx]

Fix docs and const-ness in capi. Closed by 23a5f07.