Add #![warn(missing_docs)]?

[boundless-forest]

Long-term, requiring additional documentation can be beneficial.

[AurevoirXavier]

We can start this on some of our custom pallets.

[boundless-forest]

There is a related issue darwinia-network/darwinia-docs-deprecated#304. Prefer to enhance the code docs in our customed pallet first, especially for items Const, Extrinsic, and Storage, then sync the code notes to the darwinia-docs website.

We can do this after the parachain backing and ethereum-related pallets have been cleaned up.

[hackfisher]

darwinia-common could be deprecated after darwinia 2.0.

The request is good, but should be an common practice and conversion instead.