They used to be covered by `optin_builtin_traits` but negative impls
are now applicable to all traits, not just auto traits.
This also adds docs in the unstable book for the current state of auto traits.
Previously we would only generate a list of synthetic implementations
for two well known traits – Send and Sync. With this patch all the auto
traits known to rustc are considered. This includes such traits like
Unpin and user’s own traits.
Sadly the implementation still iterates through the list of crate items
and checks them against the traits, which for non-std crates containing
their own auto-traits will still not include types defined in std/core.
It is an improvement nontheless.
The Auto Trait Implementation section is not wrapped in a
`synthetic-implementations` class. In fact, it is wrapped in a
`synthetic-implementations` id. However, we can generalize and completely remove
the `synthetic-implementations` requirement. We just have to verify that there's
no mention of "Auto Trait Implementations" anywhere.
