Backport Javadocs changes from:
LUCENE-5047: Handle NoSuchFileException of Java 7 like FileNotFoundException when opeining index files; document this in Directory

git-svn-id: https://svn.apache.org/repos/asf/lucene/dev/branches/branch_4x@1491998 13f79535-47bb-0310-9956-ffa450edef68