doc: explain lookahead behavior in sequence-generate - #5338

Welcome to Racket v8.17 [cs].
> (define inport (open-input-bytes (bytes 1 2 3 4 5 6 7 8 9 10)))
> (define-values (more? get) (sequence-generate inport))
> (more?)
#t
> (get)
1
> (more?)
#t
> (sequence-ref inport 0)
3
> (get)
2
> (more?)
#t
> (for/first ([i inport]) i)
5
> (get)
4
> (define-values (more2? get2) (sequence-generate inport))
> (more?)
#t
> (list (get2) (get))
'(7 6)
> (more2?)
#t
> (more2?)
#t
> (list (get) (get2))
'(9 8)
> (more?)
#t
> (more2?)
#f
> (more?)
#t
> (more2?)
#f