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.

git-svn-id: https://svn.apache.org/repos/asf/tomcat/trunk@1601886 13f79535-47bb-0310-9956-ffa450edef68