Fixes to FrTime dynamic value snips (for textual content, at least).

* Expect DrRacket to make a copy of the copy of the snip we give it.

* Add documentation explaining some of the trickiness involved in this code
  and how it might be possible to make it less brittle.

* Add a note about editor-snip% not behaving as expected when non-textual
  elements are inserted and subsequently removed (or the underlying
  editor object is replaced).