Add explanation why separate data struct and parsing in rustc_attr_parsing

Signed-off-by: xizheyin <[email protected]>

Author: xizheyin

compiler/rustc_attr_parsing/src/lib.rs

@@ -1,10 +1,17 @@
 //! Centralized logic for parsing and attributes.
 //!
-//! This crate is part of a series of crates that handle attribute processing:
-//! - rustc_attr_data_structures: Defines the data structures that store parsed attributes
-//! - rustc_attr_parsing: This crate, handles the parsing of attributes
+//! ## Architecture
+//! This crate is part of a series of crates that handle attribute processing.
+//! - [rustc_attr_data_structures](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_attr_data_structures/index.html): Defines the data structures that store parsed attributes
+//! - [rustc_attr_parsing](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_attr_parsing/index.html): This crate, handles the parsing of attributes
 //! - (planned) rustc_attr_validation: Will handle attribute validation
 //!
+//! The separation between data structures and parsing follows the principle of separation of concerns.
+//! Data structures (`rustc_attr_data_structures`) define what attributes look like after parsing.
+//! This crate (`rustc_attr_parsing`) handles how to convert raw tokens into those structures.
+//! This split allows other parts of the compiler to use the data structures without needing
+//! the parsing logic, making the codebase more modular and maintainable.
+//!
 //! ## Background
 //! Previously, the compiler had a single attribute definition (`ast::Attribute`) with parsing and
 //! validation scattered throughout the codebase. This was reorganized for better maintainability