Avoid conflicting anchor name in documentation

What does this MR do?

Renamed section names under only and except (complex) to include only: namespace so that the true variables section can be navigated from the right-side navigation with https://docs.gitlab.com/ce/ci/yaml/#variables.

Related issues

I have not opened an issue yet, nor can I find an existing issue on this.

The problem is, when I select variables section from the navigation on the right side, in Configuration of your jobs with .gitlab-ci.yml, the browser does not go to the section I want, because another section with the same title (thus the same HTML anchor) exists.

This MR tries to fix that by renaming one of the titles.

Author's checklist

• Apply the correct labels and milestone
• Crosslink the document from the higher-level index
• Crosslink the document from other subject-related docs
• Correctly apply the product badges and tiers
• Port the MR to EE (or backport from CE): always recommended, required when the ee-compat-check job fails

Review checklist

• Your team's review (required)
• PM's review (recommended, but not a blocker)
• Technical writer's review (required)
• Merge the EE-MR first, CE-MR afterwards