Add lint for undocumented functions

[chipshort]

We can still decide that a function is clear enough from the name and supress it explicitly, but we want to be explicit about it.

[webmaster128]

See those clippy rules:

• https://doc.rust-lang.org/rustdoc/lints.html#missing_docs
• https://doc.rust-lang.org/rustdoc/lints.html#missing_crate_level_docs
• https://doc.rust-lang.org/rustdoc/lints.html#missing_doc_code_examples
• [maybe more]