message: Document that OUT_DIR in JSON messages is an absolute path
diff: @@ -214,8 +214,8 @@ may be found in [the chapter on build scripts](build-scripts.md).
  ["SOME_KEY", "some value"],
  ["ANOTHER_KEY", "another value"]
  ],
  - /* A path which is used as a value of `OUT_DIR` environmental variable
  - when compiling current package.
  + /* An absolute path which is used as a value of `OUT_DIR` environmental
  + variable when compiling current package.
  */
  "out_dir": "/some/path/in/target/dir"
  }
  