couchdb returner: Document a likely configuration.

It is surprising to the newbie that only the first minion gets to write its data
in the couchdb database.

This documents a workaround that I hope is as clean as possible: create one db
per minion (for instance with the minion id in it).

Ref issue: #16265
Merge pull request #16288 from feth/develop

couchdb returner: Document a likely configuration.