bug#8240: mention prog-mode-hook with flyspell-prog-mode

[Chong Yidong]

jidanni@jidanni.org writes:

> At the end of (info "(emacs) Spelling")
> why don't you mention one can do
> (add-hook 'prog-mode-hook 'flyspell-prog-mode)
> to set it up in each and every program all in one fell swoop!
> Indeed the flyspell-prog-mode docstring could mention it too.

Done now, thanks for the suggestion.  Note that prog-mode is new to
Emacs 24, and the documentation is mostly not updated yet.