> I think we should add something like "if you are using generator, consider use imap instead" 

I think it's not good hint.
There are short generator.  And there are long (or infinite) iterator other than generator too.
Maybe, "if iterator is not sequence (e.g. generator) and can be very big, consider using `imap` or `imap_unorderd` with explicit `chunksize` option for better efficiency."

But I'm not good at writing English.
Someone other than me can write better paragraph.