Document all public items in rustc_incremental

Also:

- Review and edit current docs
- Enforce documentation for crate

Co-authored-by: r00ster <r00ster91@protonmail.com>
Co-authored-by: Camille Gillot <gillot.camille@gmail.com>

compiler/rustc_incremental/src/lib.rs

@@ -1,5 +1,6 @@
 //! Support for serializing the dep-graph and reloading it.
 
+#![deny(missing_docs)]
 #![doc(html_root_url = "https://doc.rust-lang.org/nightly/nightly-rustc/")]
 #![feature(in_band_lifetimes)]
 #![feature(let_else)]