Document RocksDB variables: rocksdb_max_background_compactions, rocksdb_max_background_flushes, and rocksdb_max_bottom_pri_background_compactions

   :dyn: No
   :scope: Global
   :vartype: Unsigned Integer
   :default: 0

                         "Creating specified number of threads, setting lower "
                         "CPU priority, and letting compactions use them. "
                         "Maximum compaction concurrency is capped by "
                         "rocksdb_max_background_compactions or "
                         "rocksdb_max_background_jobs."
                      min = 0,  max= 64

   :dyn: No
   :scope: Global
   :vartype: Integer
   :default: -1

"Set DBOptions::max_background_flushes for RocksDB"
                      min = -1,  max= 64

   :dyn: Yes
   :scope: Global
   :vartype: Integer
   :default: -1

"Set DBOptions::max_background_compactions for RocksDB"
                      min = -1,  max= 64