change `stream-append' to use its last argument as the result tail

Closes PR 13130

collects/racket/stream.rkt

@@ -105,6 +105,7 @@
 (define (streams-append l)
   (cond
    [(null? l) empty-stream]
+   [(null? (cdr l)) (car l)]
    [(-stream-empty? (car l)) (streams-append (cdr l))]
    [else
     (make-do-stream (lambda () #f)

collects/scribblings/reference/sequences.scrbl

@@ -717,7 +717,8 @@ A shorthand for nested @racket[stream-cons]es ending with
          stream?]{
   Returns a stream that contains all elements of each stream in the
   order they appear in the original streams.  The new stream is
-  constructed lazily.}
+  constructed lazily, while the last given stream is used in the tail of
+  the result.}
 
 @defproc[(stream-map [f procedure?]
                      [s stream?])