docs(gctx): expand module level docs for config schemas

Author: weihanglo

src/cargo/util/context/mod.rs

@@ -20,6 +20,10 @@
 //! - [`StringList`]: Get a value that is either a list or a whitespace split
 //!   string.
 //!
+//! # Config schemas
+//!
+//! Configuration schemas are defined in the [`schema`] module.
+//!
 //! ## Config deserialization
 //!
 //! Cargo uses a two-layer deserialization approach:

src/cargo/util/context/schema.rs

@@ -1,3 +1,15 @@
+//! Cargo configuration schemas.
+//!
+//! This module contains types that define the schema for various configuration
+//! sections found in Cargo configuration.
+//!
+//! These types are mostly used by [`GlobalContext::get`](super::GlobalContext::get)
+//! to deserialize configuration values from TOML files, environment variables,
+//! and CLI arguments.
+//!
+//! Schema types here should only contain data and simple accessor methods.
+//! Avoid depending on [`GlobalContext`](super::GlobalContext) directly.
+
 use std::borrow::Cow;
 use std::collections::HashMap;
 use std::ffi::OsStr;