Update config.adoc fork section

[thomastthai]

Added clarification that fork is the equivalent of the basic key checks in switch, using none of the list logic items.

Describe your changes. Use imperative present tense.

Per Discussion #1422

Checklist

• Add documentation to docs/config.adoc
  • Yes
• Add example and basic docs to cfg_samples/kanata.kbd
  • N/A
• Update error messages
  • N/A
• Added tests, or did manual testing
  • Yes

[jtroo]

Thanks!