Fix docstring for return type of educe.corpus.Reader.slurp()

[moreymat]

The docstring for educe.corpus.Reader.slurp() states it returns a dictionary from FileId to educe.annotation.Document.
However educe.rst_dt.corpus.Reader.slurp() returns a dictionary from FileId to educe.annotation.RSTTree (as correctly described by its own docstring), which is a direct subclass of educe.annotation.Standoff.
This breaks assumptions regarding the set of methods and attributes that can be called on the returned objects.

[kowey]

Oh dear, bound to be loads of these sorts of shenanigans littered throughout the API. I miss Haskell.

[kowey]

Come to think of it, maybe the right solution here is to have a more generic Document that RSTTree should be an instance of.