Explicitly mention custom params for generic alerts

What does this MR do?

Explicitly mention that a generic alert payload can contain any other extra parameters, besides the expected ones.

I caught myself on the thought, that it could be not very obvious from the generic alert docs, that on top of expected parameters, the alert's payload can contain any other custom fields. As an improvement, maybe we can be explicit about that.

Screenshots

Does this MR meet the acceptance criteria?

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