Document RocksDB variable: rocksdb_read_free_rpl.

   :dyn: Yes
   :scope: Global
   :vartype: Enum
   :default: ``OFF``

    "Use read-free replication on the slave (i.e. no row lookup during "
    "replication). Default is OFF, PK_SK will enable it on all tables with "
    "primary key. PK_ONLY will enable it on tables where the only key is the "
    "primary key (i.e. no secondary keys).",