Hide @_impl implementation detail from the bitflags! rustdoc

[dtolnay]

The rustdoc currently shows this distracting internal rule:

(@_impl flags $BitFlags:ident: $T:ty {
    $($(#[$Flag_attr:meta])* const $Flag:ident = $value:expr),+
}) => { ... };

[brson]

The way to do this is to create a new internal macro just for this case, probably with __ prefix, and mark it #[doc(hidden)].