message: Add more information to the translation pass docs
  This attempts to drive home the importance of the `surface_to_core` pass, hopefully acting as signpost for future contributors.
diff: //! Passes between intermediate languages.
  +//!
  +//! The most significant step in this process is the [`surface_to_core`] pass,
  +//! which handles elaboration of the surface language into the core language,
  +//! and is the source of most user-facing typing diagnostics.
  pub mod core_to_pretty;
  pub mod core_to_surface;
  