Polish rustdocs generated from our published crates

[nagisa]

As a minimum all published crates should contain something along the lines of

[package.metadata.docs.rs]
all-features = true
rustdoc-args = ["--cfg", "nearcore_rustdoc"]

in their Cargo.toml (alongside a themis rule to ensure this remains true) as well as doc_auto_cfg feature enabled at the very minimum (through #![cfg_attr(nearcore_rustdoc, feature(doc_{auto_,}cfg))]). Thus ensuring that the documentation is complete and the references are in place for conditionally-enabled items.