Add a document describing a solution which uses XSSI and negotiation

to custom-tailor the apache ErrorDocuments to taste, adding the
advantage of returning internationalized versions of the error
messages depending on the client's language preferences.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@80119 13f79535-47bb-0310-9956-ffa450edef68