Add comment on http_digest auth. 

I just thought it would be usefull to see what keys are needed to get HTTP Digest Auth working.
Update reference/configuration/security.rst

add empty line
Update reference/configuration/security.rst

Added PHP and XML examples.
Update reference/configuration/security.rst

Formatting fixes, added file comments.
Update reference/configuration/security.rst

fix newline
Merge pull request #2170 from tarjei/patch-1

Add comment on http_digest auth.