Add to the Java docs a .asList() vs .iterator() comparison

See https://forums.foundationdb.org/t/latency-of-range-queries-that-return-large-number-of-key-value-pairs/1291 for context.

I feel like everyone that's evaluated using FDB in Java ends up doing this comparison to figure out which function is the correct one to use to read a range of data as quick as possible. We should just add that answer to the docs of .asList() and .iterator() so that people stop having to individually write the same benchmark over and over.