Document synchronisation lock

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/pool/trunk@1622090 13f79535-47bb-0310-9956-ffa450edef68
Object is not replaced so might as well be final

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/pool/trunk@1622091 13f79535-47bb-0310-9956-ffa450edef68
Not thread-safe without volatile - needed to ensure safe publication

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/pool/trunk@1622092 13f79535-47bb-0310-9956-ffa450edef68