Terraform data resource dependencies docs are confusing or imprecise

[jbardin]

The Data Resource Dependencies section is missing from current documentation. The closest replacement is under Dependencies, but that paragraph is missing key components and is somewhat unclear.

References:

• Docs/reference rewrites phrase 1 #530 (comment)
• Avoid unnecessary data lookup deferral terraform#37761

[rkoron007]

@trujillo-adam Because you wrote this originally!