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
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