Fix Log() doc for default level
Fix Log() doc for default level

Summary: as titled

Test Plan: none

Reviewers: sdong, lightmark

Reviewed By: lightmark

Subscribers: andrewkr, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D60507

bbiao/rocksdb,OverlordQ/rocksdb,SunguckLee/RocksDB,hobinyoon/rocksdb,vmx/rocksdb,OverlordQ/rocksdb,ryneli/rocksdb,tsheasha/rocksdb,hobinyoon/rocksdb,norton/rocksdb,wenduo/rocksdb,hobinyoon/rocksdb,bbiao/rocksdb,SunguckLee/RocksDB,norton/rocksdb,facebook/rocksdb,wat-ze-hex/rocksdb,tsheasha/rocksdb,bbiao/rocksdb,ryneli/rocksdb,OverlordQ/rocksdb,bbiao/rocksdb,OverlordQ/rocksdb,vmx/rocksdb,Andymic/rocksdb,wat-ze-hex/rocksdb,jalexanderqed/rocksdb,Andymic/rocksdb,norton/rocksdb,wat-ze-hex/rocksdb,ryneli/rocksdb,wenduo/rocksdb,wenduo/rocksdb,Andymic/rocksdb,norton/rocksdb,norton/rocksdb,facebook/rocksdb,SunguckLee/RocksDB,SunguckLee/RocksDB,vmx/rocksdb,wat-ze-hex/rocksdb,jalexanderqed/rocksdb,wat-ze-hex/rocksdb,SunguckLee/RocksDB,OverlordQ/rocksdb,wenduo/rocksdb,hobinyoon/rocksdb,OverlordQ/rocksdb,Andymic/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,Andymic/rocksdb,Andymic/rocksdb,jalexanderqed/rocksdb,wat-ze-hex/rocksdb,facebook/rocksdb,tsheasha/rocksdb,norton/rocksdb,Andymic/rocksdb,SunguckLee/RocksDB,tsheasha/rocksdb,tsheasha/rocksdb,facebook/rocksdb,wenduo/rocksdb,bbiao/rocksdb,jalexanderqed/rocksdb,jalexanderqed/rocksdb,facebook/rocksdb,tsheasha/rocksdb,bbiao/rocksdb,wenduo/rocksdb,vmx/rocksdb,facebook/rocksdb,tsheasha/rocksdb,wenduo/rocksdb,jalexanderqed/rocksdb,SunguckLee/RocksDB,ryneli/rocksdb,vmx/rocksdb,norton/rocksdb,ryneli/rocksdb,hobinyoon/rocksdb,hobinyoon/rocksdb,jalexanderqed/rocksdb,vmx/rocksdb,wat-ze-hex/rocksdb,wenduo/rocksdb,hobinyoon/rocksdb,ryneli/rocksdb,tsheasha/rocksdb,ryneli/rocksdb,bbiao/rocksdb,vmx/rocksdb,facebook/rocksdb,vmx/rocksdb,bbiao/rocksdb,OverlordQ/rocksdb,wat-ze-hex/rocksdb,hobinyoon/rocksdb,facebook/rocksdb,norton/rocksdb