[Java] Improve documentation for RocksEnv and its C++ resource.

Summary:
Improve documentation for RocksEnv and its C++ resource.  Specifically,
the result of RocksEnv::Default() is a singleton, and the ownership
of its c++ resource belongs to rocksdb c++.  As a result, calling
dispose() of the return value of RocksDB::Default() will be no-op.

Test Plan: no code change.

Reviewers: haobo, ankgup87

Reviewed By: ankgup87

Subscribers: leveldb

Differential Revision: https://reviews.facebook.net/D18945