Update the summary part of mod_mime documentation to:

  - not state that extensions must be given in a particular order
  - mention assigning of handlers and use of info in content-negotiation
  - mention effect of 'unknown' extensions


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@78263 13f79535-47bb-0310-9956-ffa450edef68