ref-manual: add empty-dirs QA check and QA_EMPTY_DIRS*

This check is new in kirkstone.

(From yocto-docs rev: baa483e75538013be3261f8c115b8a5364b24c60)

Signed-off-by: Paul Eggleton <paul.eggleton@microsoft.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/ref-manual/classes.rst

@@ -1040,6 +1040,11 @@ Here are the tests you can list with the :term:`WARN_QA` and
    cases, such as dynamically loaded modules, these symlinks
    are needed instead in the main package.
 
+-  ``empty-dirs:`` Checks that packages are not installing files to
+   directories that are normally expected to be empty (such as ``/tmp``)
+   The list of directories that are checked is specified by the
+   :term:`QA_EMPTY_DIRS` variable.
+
 -  ``file-rdeps:`` Checks that file-level dependencies identified by
    the OpenEmbedded build system at packaging time are satisfied. For
    example, a shell script might start with the line ``#!/bin/bash``.