docs(renovate): say the runner serves the preset only without a file

The companion gitlab-com/public-sector/pipeline MR (branch fix/renovate-enrollment-single-load) stops injecting the shared preset as a global extends, which loaded it twice on every project whose renovate.json extends it. The runner now serves the preset only to a project with no config file.

This MR updates the two sentences in standards/renovate that described the old injection. The rule is unchanged: a file that ships extends the preset first, and that file is now the only place the preset loads from. Land it after the pipeline MR merges.

flowchart LR
  R{"renovate.json?"} -->|none| O["runner onboardingConfig: preset"]
  R -->|extends preset| F["the file's own extends: preset"]
  R -->|enabled false| D["disabled"]
  O --> L["preset loads once"]
  F --> L