Rename getDocumentSlice to cloneFromRange

We already getSlice which returns a ve.dm.DocumentSlice, so using
the word slice in this method is very confusing. What we are actually
doing is creating a ve.dm.Document from a range. Also remove argument
overloading as it's not particularly helpful and would make the new
name a lie.

Change-Id: I93da3419510410b170396e6765fbe2a87f9795be
Merge "Rename getDocumentSlice to cloneFromRange"