lint: Improve ffi-unsafe enum lint warning

I think this is an improvement of the previous warning message, which
- like the comment that I removed implies - is in need of some
improvement.
I've opted to point the user in the right direction w.r.t how to fix the
problem, which I think is good form.

Not being familiar with the repr(...) attribute, I personally had to
check the lint rules myself to figure out what was wrong. Hopefully,
this will save he next person some time and headache.

Signed-off-by: Anton Lofgren <alofgren@op5.com>
Of course, the message is only a suggestion - if someone's got something even better I'll happily change it. As long as the resulting message actually tells me what I could do to fix it, I'll be happy :-)