Clarify what max_network_retries does

[Changaco]

It's not limited to network errors, it also triggers a retry when a 409 HTTP response is returned by the API.

[brandur-stripe]

Thanks @Changaco!

Can you:

1. Replace the word "intermittent" in the first paragraph with "transient". I think this is more apt in describing things like 409s as well.
2. Let's just remove that second paragraph. I think it'd be better not to be too specific here so that we have more flexibility in updating the implementation without changing the README. A future change for example might be to detect a lock timeout and retry on that as well.
3. Squash when it's all together.

[Changaco]

@brandur-stripe I would rather not drop the second paragraph that this PR was made to add, so I reworded it instead. Let me know if it's still not good enough.

[brandur-stripe]

Okay that works.