Further fix for https://issues.apache.org/bugzilla/show_bug.cgi?id=56606
Update documentation for MemoryRealm.
Align MemoryUserRule class (in MemoryRuleSet.java) with MemoryUserCreationFactory class (in MemoryUserDatabase.java) so that "username" attribute is read first.
Merged r1601886 from tomcat/trunk.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/tc7.0.x/trunk@1601887 13f79535-47bb-0310-9956-ffa450edef68