We have two macro definitions feature gated on `serde`. At some stage we
added the `doc(hidden)` attribute to one of them but forgot to add it to
the other. This technically makes our features non-additive. This macro
is "internal" so its unlikely that this is being used in the wild.
Add `doc(hidden)` to the `serde_impl` macro that is missing it.
Found by `cargo semver-checks` after recent upgrade to 0.38