Update docs on usage of spack config vars in concrete env path

lib/spack/docs/environments.rst

@@ -494,7 +494,9 @@ from the command line.
 
 You can also include an environment directly in the ``spack.yaml`` file. It
 involves adding the ``include_concrete`` heading in the yaml followed by the
-absolute path to the independent environments.
+absolute path to the independent environments. Note, that you may use Spack
+config variables such as ``$spack`` or environment variables as long as the
+expression expands to an absolute path.
 
 .. code-block:: yaml
 
@@ -504,7 +506,7 @@ absolute path to the independent environments.
          unify: true
      include_concrete:
      - /absolute/path/to/environment1
-     - /absolute/path/to/environment2
+     - $spack/../path/to/environment2
 
 
 Once the ``spack.yaml`` has been updated you must concretize the environment to