add some pointer to determine when upgrading is complete

[TomonoriSoejima]

https://www.elastic.co/guide/en/elasticsearch/reference/7.4/rolling-upgrades.html#rolling-upgrades shows the step of rolling upgrade process, but it would be nice to add final step as to how to validate the upgrade finished successfully.

I can imagine that everyone must be issuing some flavor of cat API and it would be helpful if we can list them up in this page as well.

[elasticmachine]

Pinging @elastic/es-docs (>docs)

[elasticmachine]

Pinging @elastic/es-core-features (:Core/Features/Features)

[matriv]

@TomonoriSoejima Thank you for bringing this up to our attention.
A PR is out to provide more info on how to check the status of a rolling upgrade.

[matriv]

master : 8ca11f5
7.x : d5842ae
7.5 : eefc6ec
7.4 : 073449b
6.8 : 6e643fe