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

[jidanni]

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.