Clarify doc string of 'transpose-sexps'

* lisp/simple.el (transpose-sexps): Clarify the conditions for
transposing sexps that are lists or strings.  Mention the effect
of the prefix argument.  (Bug#24860)

lisp/simple.el

@@ -6663,9 +6663,13 @@ are interchanged."
   (transpose-subr 'forward-word arg))
 
 (defun transpose-sexps (arg)
-  "Like \\[transpose-words] but applies to sexps.
-Does not work on a sexp that point is in the middle of
-if it is a list or string."
+  "Like \\[transpose-chars] (`transpose-chars'), but applies to sexps.
+Unlike `transpose-words', point must be between the two sexps and not
+in the middle of a sexp to be transposed.
+With non-zero prefix arg ARG, effect is to take the sexp before point
+and drag it forward past ARG other sexps (backward if ARG is negative).
+If ARG is zero, the sexps ending at or after point and at or after mark
+are interchanged."
   (interactive "*p")
   (transpose-subr
    (lambda (arg)