Docs for `Group` type

[Bahman Movaqar via Digitalmars-d-learn]

When using `chunkBy` (unary pred) the result is a list of tuples.  Each
tuple holds a key and a `Group` which belong to that key.
Where can I find the docs for this `Group` type (I have already tried
searching library on dlang.org)?

Thanks,
-- 
Bahman