Doc about InstallTracker files and install.root

Author: weihanglo

src/doc/src/guide/cargo-home.md

@@ -18,8 +18,8 @@ The Cargo home consists of following components:
 * `credentials.toml`
  	Private login credentials from [`cargo login`] in order to log in to a [registry][def-registry].
 
-* `.crates.toml`
-	This hidden file contains [package][def-package] information of crates installed via [`cargo install`]. Do NOT edit by hand!
+* `.crates.toml`, `.crates2.json`
+	These hidden files contain [package][def-package] information of crates installed via [`cargo install`]. Do NOT edit by hand!
 
 ## Directories:
 

src/doc/src/reference/config.md

@@ -612,6 +612,9 @@ The `[install]` table defines defaults for the [`cargo install`] command.
 Sets the path to the root directory for installing executables for [`cargo
 install`]. Executables go into a `bin` directory underneath the root.
 
+To track information of installed executables, some extra files, such as
+`.crates.toml` and `.crates2.json`, would also be created under this root.
+
 The default if not specified is Cargo's home directory (default `.cargo` in
 your home directory).