Update 'Configure Federation' documentation.

Apache 2.4 becomes more and more popular version of HTTP server.
With its release some options required for proper Shibboleth Service
Provider have changed. This patch updates information about ``ShibRequireAll``
options that is no longer valid under Apache 2.4. It also adds
documentation reference so users can read more about configuring Apache
Virtual Hosts along with Shibboleth Service Provider.

Change-Id: Idbec6690fd1d75f078a6bdcaa6388b885387b45d
Merge "Update 'Configure Federation' documentation."