bug#8151: add flyspell-prog-mode initiation example to Info

[jidanni]

In (info "(emacs) Spelling")

   Type `M-x flyspell-mode' to enable or disable this mode in the
   current buffer. To enable `flyspell-mode' in all text mode buffers,
   add `turn-on-flyspell' to `text-mode-hook'. ...

      Flyspell Prog mode works just like ordinary Flyspell mode, except
   that it only checks words in comments and string constants.  This
   feature is useful for editing programs.  Type `M-x flyspell-prog-mode'
   to enable or disable this mode in the current buffer.

OK, but the second paragraph doesn't say what hook to turn
flyspell-prog-mode on for all programs!

Or mention how to turn it on one by one...

Well it turns out one needs to use a totally different method to turn on
flyspell-prog-mode, e.g.,
(add-hook 'perl-mode-hook
  (function (lambda ()(flyspell-prog-mode))))
And it turns out there is no way to turn off flyspell-prog-mode... OK,
probably just turn off flyspell-mode.

Also in the mode line there is no distinction of the two types of
flyspell-mode... probably because no big deal.