Document how to use CustomizeDiff in MMv1

[melinath]

What kind of contribution is this issue about?

MMv1-based resource

Details

In MMv1, you can add CustomizeDiff functions with the custom_diff attribute in YAML. However, this is not currently documented anywhere. We should document it on the MMv1 resource reference (similar to https://googlecloudplatform.github.io/magic-modules/develop/field-reference/#diff_suppress_func), as well as mention cases when it should be used on Fix a permadiff.

References

No response

[melinath]

Hashicorp documentation is at https://developer.hashicorp.com/terraform/plugin/sdkv2/resources/customizing-differences - we may want to be a bit more involved than what's laid out there.

[melinath]

We should explicitly document how to use it for cross-field validation.