flatten docstring does not describe lazy result

clojure.core/flatten uses tree-seq to return a lazy result. The lazy nature of the result is not described in the docstring.

Original docstring:
-------------------------------- "Takes any nested combination of sequential things (lists, vectors,
etc.) and returns their contents as a single, flat sequence.
(flatten nil) returns an empty sequence."

Proposed docstring:
-------------------------------- "Takes any nested combination of sequential things (lists, vectors,
etc.) and returns their contents as a single, flat lazy sequence.
(flatten nil) returns an empty sequence."

Screened by: Alex Miller