Improve instructions about federation

This patch clarifies two elements:

1. According to Shibboleth's documentation the ShibRequireAll command
is not supported on Apache 2.4+, details can be found at
https://wiki.shibboleth.net/confluence/display/SHIB2/NativeSPApacheConfig

2. Some configurations of Apache + Shibboleth SP will always set the
REMOTE_USER env variable, even if it is an empty value; this can
lead to unexplained authentication errors that can be avoided if the
external auth method is disabled.

Change-Id: Iaf4e57e7e3d79a2e87e805bbe8340861ee9c3db3
Merge "Improve instructions about federation"