Docs: Clarify namespace on Terraform docs

Problem to solve

Resolve https://docs.gitlab.com/ee/user/packages/terraform_module_registry/index.html#comment-5708862024:

• What is the namespace when sourcing a module?
• What if the module repo is in multiple groups?

Proposal

We need an eng to clarify this on the docs.

Who can address the issue

An eng that belongs to ~"group::configure".

Other links/references

Slack thread: https://gitlab.slack.com/archives/C7WE1CBQW/p1643274523004700?thread_ts=1643219448.004500&cid=C7WE1CBQW