message: Add documentation for JSON message_path.
diff: @@ -67,6 +67,8 @@ structure:
  "reason": "compiler-message",
  /* The Package ID, a unique identifier for referring to the package. */
  "package_id": "my-package 0.1.0 (path+file:///path/to/my-package)",
  + /* Absolute path to the package manifest. */
  + "manifest_path": "/path/to/my-package/Cargo.toml",
  /* The Cargo target (lib, bin, example, etc.) that generated the message. */
  "target": {
  /* Array of target kinds.
  @@ -129,6 +131,8 @@ following structure:
  "reason": "compiler-artifact",
  /* The Package ID, a unique identifier for referring to the package. */
  "package_id": "my-package 0.1.0 (path+file:///path/to/my-package)",
  + /* Absolute path to the package manifest. */
  + "manifest_path": "/path/to/my-package/Cargo.toml",
  /* The Cargo target (lib, bin, example, etc.) that generated the artifacts.
  See the definition above for `compiler-message` for details.
  */
  