Merge branch 'jk/doc-http-backend' into maint

Improve documentation to illustrate "push authenticated, fetch
anonymous" configuration for smart HTTP servers.

* jk/doc-http-backend:
  doc/http-backend: match query-string in apache half-auth example
  doc/http-backend: give some lighttpd config examples
  doc/http-backend: clarify "half-auth" repo configuration