What is "FilterResult" type?

[Bahman Movaqar via Digitalmars-d-learn]

From what I can gather the output of `std.algorithm.iteration : 
filter` is a `FilterResult` type.
I need a bit of help dealing with this type:
  1. Why this type is there in the first place instead of simply 
using the type of input range?
  2. Where is the documentation for this type?  The documentation 
always uses `auto` for `filter` return values.
  3. How can I convert this type back into a range?
Thanks.