CTR: docs.
https://issues.apache.org/bugzilla/show_bug.cgi?id=56606
Update documentation for MemoryRealm to recommend the "username" attribute.
It is partial backport of r1601886 from tomcat/trunk.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc6.0.x/trunk@1601892 13f79535-47bb-0310-9956-ffa450edef68