Improve byChunk, lockingTextWriter doc

Make `byChunk`, `lockingTextWriter` return opaque types (but keep those types public for backward compatibility). This is consistent with `byLine`'s recent change in pull 1408 - see discussion:
https://github.com/D-Programming-Language/phobos/pull/1408#issuecomment-20759752

Add `byChunk` range example.
Also document that `ByLine`'s primitives can throw `StdioException`.