generator documentation improvement

[Paul Pogonyshev <pogonyshev@gmail.com>]

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I suggest the following short improvement to documentation of
generator functions.  It took me very long to understand what's wrong
with my code.

Problem can be illustrated with this:

(require 'generator)
(defvar xxx 0)
(iter-defun my-iter ()
  (print "A")
  (let ((xxx nil))
    (print "B")
    (iter-yield 0)
    (print "C"))
  (print "D"))
(iter-do (x (my-iter))
  (print x))

If you evaluate the last form, you'll see "B" and "C" printed twice
because `let' for dynamic variable is internally implemented with
something similar to `unwind-protect'. However, when documentation
doesn't mention it, that's very hard to pinpoint.

Paul

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diff --git a/doc/lispref/control.texi b/doc/lispref/control.texi
index 401a999cf2..e279be8557 100644
--- a/doc/lispref/control.texi
+++ b/doc/lispref/control.texi
@@ -730,7 +730,8 @@ result as its condition data.
 
 Any kind of Lisp code is valid inside @var{body}, but
 @code{iter-yield} and @code{iter-yield-from} cannot appear inside
-@code{unwind-protect} forms.
+@code{unwind-protect} forms or any dynamic @code{let} bindings,
+i.e. bindings for variables defined with @code{defvar}.
 
 @end defmac