Minor update in Security user guide chapter.

- Clarified how RolesAllowedFeature can be registered when extending ResourceConfig (for Servlet deployments).

Change-Id: I78f2f005a77ca308546bf0661d0bc67a647aab79
Signed-off-by: Marek Potociar <marek.potociar@oracle.com>
Merge "Minor update in Security user guide chapter."