Restrictions on interface methods should be clarified

[ericlippert]

The text of chapter 17 on interface methods says

"Methods declared in an interface must not be declared abstract."

That's correct but incomplete. We should add to this:

• Methods declared in an interface may be static.

This is good to point out because many languages do not allow static interface methods.

• Methods declared in an interface must not be declared protected or private.

Clearly it makes no sense to have a protected or private interface member; they're implicitly public.

• Methods declared in an interface may be marked public.

HHVM allows the "public" to be elided, even though technically that is not quite grammatical. I think that's a reasonable rule.