document that RocksDB-based manysearch does not load all query sketches into memory!!!

[ctb]

it's kind of cool 😆

see

sourmash_plugin_branchwater/src/mastiff_manysearch.rs

Line 57 in a45163e

let send_result = query_collection

[ctb]

neither does mastiff_manygather!

[ctb]

since I came back to this and was wondering why I was so excited and it took me a minute: when using regular collections, manysearch loads all the query sketches into memory. Which, if you want to do a large search of many queries, can be a lot. But it doesn't do with a RocksDB index.