docs for support of env vars in cache_dir

Author: RockBomber

doc/en/customize.rst

@@ -269,5 +269,6 @@ Builtin configuration file options
    Sets a directory where stores content of cache plugin. Default directory is
    ``.cache`` which is created in :ref:`rootdir <rootdir>`. Directory may be
    relative or absolute path. If setting relative path, then directory is created
-   relative to :ref:`rootdir <rootdir>`. For more information about cache plugin
+   relative to :ref:`rootdir <rootdir>`. Additionally path may contain environment
+   variables, that will be expanded. For more information about cache plugin
    please refer to :ref:`cache_provider`.