[doc] explicitly mention how to be backwards compatible with v2 client when upgrading from prior 3.4 versions.

[guihao-liang]

This is not a bug but a request. I know etcd3 uses grpc but it still supports v2 restful. But after 3.4, it disables v2 by default. This is a huge breaking change. Could you emphasize this in https://etcd.io/docs/v3.4.0/?

It's hard to find this change unless read the change log and upgrade notes from 3.1 all along to 3.4 or follow the releases all the time.

I'm coming from 3.1 to 3.4 and new to etcd. It's very time consuming for me to read all change notes from 3.1 to 3.4 to get this breaking change. If I don't read upgrade notes from 3.3 to 3.4, I would never know this change and it indeed breaks my pipeline and took me a long day to figure out since I'm using python-etcd, which only supports v2.

I think this is true for others who are shifting from non-3.3 versions to 3.4 and uses v2 client, they wouldn't know this breaking change neither until their pipelines hit the similar issue as I did.

proposal

• have a standalone page to describe all API breaking change and ping it at the top of the documentation.
• in developer guide, have a backwards-compatible page mentioning how should people upgrade their etcd while holding v2 client unaffected.

[stale]

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