chore: Add databaseIndex to documentation of redis-cluster scaler

[peschmae]

Provide a description of what has been changed

Add databaseIndex argument to documentation of redis-cluster scaler.
The text is identical to the redis scaler.

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Fixes #

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[JorTurFer]

Nice!
You're right, all the redis list/stream scalers use the same library under the hood, so the behaviour should be the same. Could you update other redis docs with this change too?

[peschmae]

I just realised that there are mutliple version directories for the docs.
I've updated 2.12 in the previous commit. Should I also do 2.13 or are some changes synced between the versions?

[peschmae]

I've squashed all doc changes for 2.12 into a single commit, and made the same changes to the docs for 2.13.

I hope it's fine like this

[JorTurFer]

LGTM! thanks for the improvement ❤️