Add a docstring showing supported usage of Pipe()

It was brought up on discourse that this function doesn't have any docs.

base/stream.jl

@@ -552,6 +552,22 @@ mutable struct Pipe <: AbstractPipe
     in::PipeEndpoint # writable
     out::PipeEndpoint # readable
 end
+
+"""
+Construct an uninitialized Pipe object.
+
+The appropriate end of the pipe will be automatically initialized if
+the object is used in process spawning. This can be useful to easily
+obtain references in process pipelines, e.g.:
+
+```
+julia> err = Pipe()
+
+# After this `p` will be initialized and you may read `foo`'s
+# stderr from the `err` pipe.
+julia> spawn(pipeline(pipeline(`foo`, stderr=err), `cat`)
+```
+"""
 Pipe() = Pipe(PipeEndpoint(), PipeEndpoint())
 pipe_reader(p::Pipe) = p.out
 pipe_writer(p::Pipe) = p.in