Mention limitation of jobs table wrt proxies

[danielmewes]

Queries run through a RethinkDB proxy currently don't show up in the jobs system table.

We should mention that limitation in https://rethinkdb.com/docs/system-jobs/ .

We plan to change this eventually with https://github.com/rethinkdb/rethinkdb/issues/5428 , but it's at least two major releases away.

[danielmewes]

We're actually going to resolve this now with RethinkDB 2.4. So maybe it's not worth putting time into.