Properly document that group-level environment-specific variables are a premium-only feature

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Problem to solve

From the implementation of #2874 (closed):

• There's no hint on the environment scope section of the variable editor when using the free edition (per the original issue description: "In FOSS, show * as the environment scope as disabled to provide a "hint" that upgrading will provide this feature.").
• https://docs.gitlab.com/ee/ci/variables/README.html#group-cicd-variables should have this explicitly marked as a premium-only feature if possible (it's hidden in the collapsed "version history" disclosure, which isn't an obvious place to look).