Add capture_docs attribute

[ascjones]

Annotating a type with #[scale_info(capture_docs = "always|never|default")] will override the default docs capturing enabled by the docs feature.

#[scale_info(capture_docs = "never")]

Never capture docs for a specific type even if the docs feature is enabled.

#[scale_info(capture_docs = "always")]

Always capture docs for a specific type even if the docs feature is not enabled.

#[scale_info(capture_docs = "default")]

Captures docs iff the docs feature is enabled, which is the default if this attribute is not present

This will allow e.g. including required docs for enums such as Call, Error etc in substrate pallets without adding docs for every single other type, thus bloating the runtime size.

Derive refactoring

Much of the changes in the diff are because of the TypeInfoImpl type I have introduced into the derive implementation, which allows sharing of the attributes and other state to avoid it having to be passed around through all the methods. We could do some further refactoring here but this is a good first step.