documentation for the regex based exclusion of 404 errors with MonologBundle
(cherry picked from commit aea65332045da444e3d39365952d5b02dc97cc5f)
[#3203] Minor tweaks to new 404 logging entry
(cherry picked from commit 6ec097d42664633457e3baf6c51c72c4fa78b555)
[#3203] Changing my mind and backporting these docs to 2.3