Document stub behavior on `clone` and `dup`.

For more details on the inner workings on `clone` vs `dup` see:

  http://stackoverflow.com/a/10183477/29262

`dup` does not copy object state. Stubs are viewed as part of object
state, so they are not transfered. However, `clone` should copy an
object's state; thus the stubs should also be copied.
Merge pull request #631 from cupakromer/dup-clone-stubs

Document stub behavior on `clone` and `dup`.