docs for ini option cache_dir

doc/en/cache.rst

@@ -1,3 +1,5 @@
+.. _`cache_provider`:
+
 Cache: working with cross-testrun state
 =======================================
 

doc/en/customize.rst

@@ -262,3 +262,14 @@ Builtin configuration file options
 
    This tells pytest to ignore deprecation warnings and turn all other warnings
    into errors. For more information please refer to :ref:`warnings`.
+
+.. confval:: cache_dir
+
+   .. versionadded:: 3.2
+
+   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>`. Additionally path may contain environment
+   variables, that will be expanded. For more information about cache plugin
+   please refer to :ref:`cache_provider`.