Improve docs feature

[ascjones]

Currently the docs feature only excludes docs for derived TypeInfo. We should also exclude docs added from manual impls via the docs builder methods.

As suggested by @thiolliere paritytech/substrate#8615 (comment) the docs methods could be inlined and made a no-op if the docs feature is disabled. We would need to double check that this works as intended by not including the static strings in the resulting Wasm.

A similar technique could also be used together with this in frame-metadata, to remove all docs. See paritytech/frame-metadata#6.