When the user want to read the manuals, they build their manuals file authentication-settings.html and find out that the description about authentication settings dosent't tell them what to do if they can't login successfully.
I have add extra lines in the authentication-settings.rst file, and after the user build the manuals, and if they can't login successfully, they will know what to do through from the authentication-settings.html file.

I've build the manual docs using make html and get the _build/html/admin/configuration/authentication-settings.html and the format of the html file looks just good to me.