Document wild impl blocks in the module where they are defined

Frans Skarmanrequested to merge
wild_traits into main

Merging into !645 (merged) since this is built on top

Author checklist

  • New Diagnostics have at least one snapshot test that triggers it
  • Added a changelog entry, if relevant
    • You can use ./add_changelog <MR number or a_descriptive_name> to do this

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