Clarify notes for terraform endpoint attribute

What does this MR do and why?

Clarifies the module_namespace attribute notes.

How to set up and validate locally

Tested in here https://gitlab.com/gitlab-gold/anton257/tickets/zd-334267-terraform-registry

TEST:

• Incorrect response - open browser at https://gitlab.com/api/v4/packages/terraform/modules/v1/gitlab-gold/anton257/tickets/my-module/local/versions
• Correct response - open browser at https://gitlab.com/api/v4/packages/terraform/modules/v1/gitlab-gold/my-module/local/versions

So even the project with terraform module belongs to the subgroup anton257, the API requires top-level gitlab-gold namespace in the endpoint address.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.