Document tuning ingress configuration to handle client upload bandwidth issues

[pontus]

Bigpicture recently worked around an issue for uploads where the client saw 500 or 502 errors while uplaoding. We currently believe the problems was bandwidth between the client and the s3proxy.

Changing the ingress configuration from streaming to the backend (s3proxy) to buffering the body in nginx before calling on the s3proxy have helped with this.

We should document this as a possible workaround for that kind of problem.