Documentation: devres: add missing IO helper

Add missing devm_request_free_mem_region() to devres.rst.
It's introduced by commit 0092908d

 ("mm: factor out a
devm_request_free_mem_region helper").

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://lore.kernel.org/r/20220927080215.1359979-1-yangyingliang@huawei.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>