Suggest users to remove REMOTE_USER from shibd conf

Shibboleth configuration docs should suggest users to avoid setting
REMOTE_USER environment variable in the shibboleth2.xml config file.

Change-Id: I11ee773dd20375c46e40d70ad09bf0a88182502e
Closes-bug: #1320128
Merge "Suggest users to remove REMOTE_USER from shibd conf"