Auto-generated docs prevent missing_docs lint

[ggwpez]

Pallet storage items got some auto-generated docs introduced here.
This makes deny(missing_docs) useless, since there are always the auto-generated docs. For example a map has this:
Storage type is [StorageMap] with key type u8 and value type u16.

I think we should either:

• remove the auto-generated docs
• only generate them when there already are docs
• emit a warning from the pallet macro if there are no docs but still auto-generate some
• create a phony public type that has the same docs and span as the original storage item, so that the lint still works. This way we can keep the auto-generation but also have the lint still work.

[ggwpez]

cc @wentelteefje