Merge r1516872, 1542353 and 1580895 from trunk:

  howto/index.xml:
    CGI is no longer the most common way to provide dynamic content, is it?
  howto/access.xml:
    Suggest the 'require expr' syntax as an alternate way to do something.
  caching.xml:
    elaborate on the early/late vs "Authorization" header (vs. other forms of 
    authorization)


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1597081 13f79535-47bb-0310-9956-ffa450edef68