readahead options have inconsistent and unclear documentation

[pdillinger]

Both adaptive_readahead and readahead_size describe RocksDB behaviors, but not how that option (or others) affects that behavior. The comment for adaptive_readahead refers to a non-existent option max_auto_readahead_size. I don't see a relevant wiki page for a high-level coherent story. It is not clear how auto_readahead_size, adaptive_readahead, and readahead_size interact.