Add exit code information to cli docs

[jamesgeorge007]

Closes #5896

Add CLI existing status information the the docs.

• I’ve added tests to confirm my change works.
• (If changing the API or CLI) I’ve documented the changes I’ve made (in the docs/ directory)
• (If the change is user-facing) I’ve added my changes to the CHANGELOG.unreleased.md file following the template.
• I’ve read the contributing guidelines.

[jamesgeorge007]

@j-f1 Thoughts 🤔

[alexander-akait]

Please fix linting problems

 prettylint {.,docs,website,website/blog}/*.md

  docs/cli.md:57:21
  ⚠  57:21  Insert ························                                                            prettier/prettier
  ⚠  58:9   Insert ------------------------                                                            prettier/prettier
  ⚠  59:3   Replace ·|·Everything·formatted·properly· with ····|·Everything·formatted·properly·······  prettier/prettier
  ⚠  60:4   Insert ···                                                                                 prettier/prettier
  ⚠  61:4   Replace |·Something's·wrong·with·Prettier with ···|·Something's·wrong·with·Prettier····    prettier/prettier

  5 warnings

[j-f1]

Thanks for contributing to Prettier!