Fix doc attributes for `mock` submodule in `client` crate

[hdevalence]

The client crate has a mock submodule that's feature gated and used for mocking. There's a way to get Rustdoc to render modules that are only available with certain feature flags, and we should use it so it shows up in the docs. Otherwise the API surface is very confusing.

[plaidfinch]

There's examples of how to do this in the monorepo in the component crate.