update docs to point to where params definitions should be per dataource

[modular-magician]

Fixes hashicorp/terraform-provider-google#10791

We probably don't want to put an example of each type, that would be too many, but updating the documentation here should help by pointing to the correct part of Google's documentation on how to define the params block.

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Derived from GoogleCloudPlatform/magic-modules#5629