Add asserts in Copy (and remove an enforce)

Adding extra asserts always helps. As for the enforce, I'm unsure why it was there in the first place: All the other branches simply make the assumption the target is large enough.
Clarify DDoc of hasElaborateAssign

Split the overly long sentence in two, otherwise
it's easy to miss the point about auto-generation.
Merge pull request #2056 from blackwhale/elaborate-assign

Clarify DDocs of hasElaborateAssign
Merge pull request #2052 from monarchdodra/copyAssert

Add asserts in Copy (and remove an enforce)