Added javadocs to *Ref about clone() performing a shallow copy. I know, this is obvious and to be expected, but I fell a victim to this anyway, it's a damn thing to debug if you use clone() in hash maps etc. so perhaps it'll help somebody.

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1483682 13f79535-47bb-0310-9956-ffa450edef68