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>
auto merge of #15893 : catharsis/rust/ffi-unsafe-enum-lint-warning, r=alexcrichton

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>