Update REST Services retry logic

[jnm]

docs/source/rest_services.md

Lines 14 to 17 in 1da489a

	In case of a failure, the background task will retry 3 times to send the data
	(first time after 60 seconds, second time after 600 seconds, and third time
	after 6,000 seconds). Email notifications can be enabled to receive a failure
	report.

We need to update this documentation to say that KoboToolbox automatically retries requests to external servers only when one of the following errors is returned by the external server (ref):

• HTTP 502 Bad Gateway
• HTTP 503 Service Unavailable
• HTTP 504 Gateway Timeout

All other failures require the user to retry manually.

Additionally, the automatic retry schedule is now different and much shorter: the retries happen after (approximately) 60, 120, and 240 seconds (ref).