Draft: Clarify conditions for measuring lt4c

What does this MR do and why?

I raised a support case #493091 to investigate why LT4C was not being measured on a project using a protected branch production for deploying to the production environment. I learned that LT4C is only calculated when a MR is merged to the default branch and the default branch is used to deploy to the production environment.

The current explanation is technically accurate, but could be a lot clearer about what conditions need to be met for GitLab to measure LT4C. This MR attempts to clarify by mentioning the environment:name and environment:deployment_tier keywords needed, along with the MR being to the project's default branch.

MR acceptance checklist

I don't think this warrants a changelog, since it updates the description of a currently released feature for clarity.

Screenshots or screen recordings

N/A

How to set up and validate locally

N/A