Suppress missing docs warning

[billy1624]

Hi, there. This PR aims to suppress missing docs warning of Rust linter while using #[ouroboros::self_referencing] with #![warn(missing_docs)] enabled.

E.g. https://github.com/SeaQL/sea-orm/runs/4076651605

missing documentation for a struct
warning: missing documentation for a struct
  --> src/database/stream/transaction.rs:14:1
   |
14 | #[ouroboros::self_referencing]
   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
   = note: this warning originates in the attribute macro `ouroboros::self_referencing` (in Nightly builds, run with -Z macro-backtrace for more info)

Thanks!!

[billy1624]

Ping @joshua-maros