Update the ErrorDocument information to clarify a few things,

especially the handling of 401 responses.  The example that was in
the docs before would never work!  No wonder people have trouble
with it...


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@77838 13f79535-47bb-0310-9956-ffa450edef68