document when no hook is available

[jidanni]

In (info "(emacs) Specifying File Variables")
at

       Often, however, it is a mistake to enable minor modes in file local
    variables.  Most minor modes, like Auto Fill mode, represent individual
    user preferences.  If you want to use a minor mode, it is better to set
    up major mode hooks with your init file to turn that minor mode on for
    yourself alone (*note Init File::), instead of using a local variable
    list to impose your taste on everyone.

please also mention what to do in ones init file when there is no such
hook available.

E.g., the user wants all ZZZ mode files to be in flyspell-prog-mode. The
problem is the ZZZ-mode author didn't remember to add a hook, and we are
on a read only system where all we can change is our .emacs.