Clarify documentation for magic activation of advice

[Markus Rost]

The file advice.el calls (ad-start-advice) at the very end.  I suggest
to document this.  The current documentation

(Info-goto-node "(elisp)Activation of Advice")

indicates that (ad-start-advice) is not called by default.  Here is an
experiment with somewhat surprising results, at least after reading
the current documentation.

(progn
  (defadvice foo (around return-t) "Return t." (setq ad-return-value t))
  (defun foo nil "Return nil." nil)
  (foo))

==> t

(progn
  (defun foo1 nil "Return nil." nil)
  (defadvice foo1 (around return-t) "Return t." (setq ad-return-value t))
  (foo1))

==> nil
==> t     (if you repeat the evaluation)


And here is a suggestion for the documentation.

        * advice.texi (Activation of Advice): Note that ad-start-advice
        is turned on by default.

===Buffer *vc-diff*=========================================
*** advice.texi.~1.15~  Tue Jun 24 14:34:23 2003
--- advice.texi Wed Jul  2 15:26:31 2003
***************
*** 331,337 ****
  @cindex advice, activating
  
  By default, advice does not take effect when you define it---only when
! you @dfn{activate} advice for the function that was advised.  You can
  request the activation of advice for a function when you define the
  advice, by specifying the @code{activate} flag in the @code{defadvice}.
  But normally you activate the advice for a function by calling the
--- 331,339 ----
  @cindex advice, activating
  
  By default, advice does not take effect when you define it---only when
! you @dfn{activate} advice for the function that was advised.  However
! the advice will be automatically activated when the function is
! defined or redefined later.  You can
  request the activation of advice for a function when you define the
  advice, by specifying the @code{activate} flag in the @code{defadvice}.
  But normally you activate the advice for a function by calling the
***************
*** 416,423 ****
  
  @deffn Command ad-start-advice
  Turn on automatic advice activation when a function is defined or
! redefined.  If you turn on this mode, then advice takes effect
! immediately when defined.
  @end deffn
  
  @deffn Command ad-stop-advice
--- 418,424 ----
  
  @deffn Command ad-start-advice
  Turn on automatic advice activation when a function is defined or
! redefined.  This mode is turned on by default.
  @end deffn
  
  @deffn Command ad-stop-advice
============================================================