add docs for playing nice with couchdb

Yes! Well, sort of. I wrote one about 2 years ago that doesn't support chunking, just saving everything in one enormous post: https://github.com/dimagi/dimagi-utils/blob/cd800b33a57ec9f082738cc6c45afa946018aa21/dimagi/utils/couch/bulk.py#L8-8. It's essentially exactly the syntax you suggested except that it's called `CouchTransaction`. As far as I know, it's only used for fixtures.