I'm not sure the "By design…" sentence and its parenthetical are very clear.  What about something along the lines of

> Because, by design, `nsqlookupd` instances don't coordinate their lists of producers, the client library should union the lists of producers received from all `nsqlookupd` queries to build the final list of `nsqd` producers to connect to.

?

The "by design" aside could even link to other docs/blog posts explaining this design decision.