update definitions

Author: mark-i-m

src/appendix/code-index.md

@@ -7,14 +7,14 @@ compiler.
 Item            |  Kind    | Short description           | Chapter            | Declaration
 ----------------|----------|-----------------------------|--------------------|-------------------
 `BodyId` | struct | One of four types of HIR node identifiers | [Identifiers in the HIR] | [src/librustc/hir/mod.rs](https://doc.rust-lang.org/nightly/nightly-rustc/rustc/hir/struct.BodyId.html)
-`CodeMap` | struct | The CodeMap maps the AST nodes to their source code | [The parser] | [src/libsyntax/codemap.rs](https://doc.rust-lang.org/nightly/nightly-rustc/syntax/codemap/struct.CodeMap.html)
+`CodeMap` | struct | Maps AST nodes to their source code. It is composed of `FileMap`s | [The parser] | [src/libsyntax/codemap.rs](https://doc.rust-lang.org/nightly/nightly-rustc/syntax/codemap/struct.CodeMap.html)
 `CompileState` | struct | State that is passed to a callback at each compiler pass | [The Rustc Driver] | [src/librustc_driver/driver.rs](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_driver/driver/struct.CompileState.html)
 `ast::Crate` | struct | A syntax-level representation of a parsed crate | [The parser] | [src/librustc/hir/mod.rs](https://doc.rust-lang.org/nightly/nightly-rustc/syntax/ast/struct.Crate.html)
 `hir::Crate` | struct | A more abstract, compiler-friendly form of a crate's AST | [The Hir] | [src/librustc/hir/mod.rs](https://doc.rust-lang.org/nightly/nightly-rustc/rustc/hir/struct.Crate.html)
 `DefId` | struct | One of four types of HIR node identifiers | [Identifiers in the HIR] | [src/librustc/hir/def_id.rs](https://doc.rust-lang.org/nightly/nightly-rustc/rustc/hir/def_id/struct.DefId.html)
 `DiagnosticBuilder` | struct | A struct for building up compiler diagnostics, such as errors or lints | [Emitting Diagnostics] | [src/librustc_errors/diagnostic_builder.rs](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_errors/struct.DiagnosticBuilder.html)
 `DocContext` | struct | A state container used by rustdoc when crawling through a crate to gather its documentation | [Rustdoc] | [src/librustdoc/core.rs](https://github.com/rust-lang/rust/blob/master/src/librustdoc/core.rs)
-`FileMap` | struct | Part of the `CodeMap`, mapping a single source file | [The parser] | [src/libsyntax_pos/lib.rs](https://doc.rust-lang.org/nightly/nightly-rustc/syntax/codemap/struct.FileMap.html)
+`FileMap` | struct | Part of the `CodeMap`. Maps AST nodes to their source code for a single source file | [The parser] | [src/libsyntax_pos/lib.rs](https://doc.rust-lang.org/nightly/nightly-rustc/syntax/codemap/struct.FileMap.html)
 `HirId` | struct | One of four types of HIR node identifiers | [Identifiers in the HIR] | [src/librustc/hir/mod.rs](https://doc.rust-lang.org/nightly/nightly-rustc/rustc/hir/struct.HirId.html)
 `NodeId` | struct | One of four types of HIR node identifiers. Being phased out | [Identifiers in the HIR] | [src/libsyntax/ast.rs](https://doc.rust-lang.org/nightly/nightly-rustc/syntax/ast/struct.NodeId.html)
 `ParamEnv` | struct | Information about generic parameters or `Self`, useful for working with associated or generic items | [Parameter Environment] | [src/librustc/ty/mod.rs](https://doc.rust-lang.org/nightly/nightly-rustc/rustc/ty/struct.ParamEnv.html)