Adds a type of output to Examples that allows tests to have unordered
output. This is intended to help clarify when the output of a command
will produce a fixed return, but that return might not be in an constant
order.

Examples where this is useful would be documenting the rand.Perm()
call, or perhaps the (os.File).Readdir(), both of which can not guarantee
order, but can guarantee the elements of the output.

Fixes #10149

Change-Id: Iaf0cf1580b686afebd79718ed67ea744f5ed9fc5
Reviewed-on: https://go-review.googlesource.com/19280Reviewed-by: Andrew Gerrand <adg@golang.org>