Improve documentation / samples for using count with data sources

[jsteinich]

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Description

Currently using count to support values that are only known by consulting a data source requires a bunch of complicated overrides. Hopefully that can be improved in the future, but in the meantime, we can improve documentation / samples.

References

https://discuss.hashicorp.com/t/how-to-access-data-list-attribute-with-python-within-resource/38115/10