Add maximum length information to changes/exists

What does this MR do?

A customer ran into the issues linked below, based on which there is a limit of 50 file paths in place that is currently not documented. There's a simple workaround if you need more, see this comment for an illustrating example.

While adding this limit to the appropriate places, I noticed that the 10000 executions limitation was mentioned in a way that could create some confusion, so I reworded that as well.

Related issues

rules:changes and rules:exists list length erro... (#216189 - closed) Backend: Permit `rules:changes` and `rules:exis... (#216196)

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