[DOC] big update to the configuration manual

[DOC] big update to the configuration manual

New in this version is a small intro to HTTP, then a detailed
explanation of the following keywords :

  acl, appsession, balance, bind, block, capture cookie,
  capture request header, capture response header, clitimeout,
  contimeout, cookie, default_backend, disabled, enabled, errorfile,
  http-check disable-on-404, monitor fail, option contstats,
  timeout client, timeout clitimeout, timeout connect,
  timeout contimeout.

Others will be alphabetically added.