[docs] Add result message examples to verifiers

[esunar]

The checks are listed in the "verifiers" section, but do not provide examples of possible results and an explanation of what they mean.

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Imported from Launchpad using lp2gh.

• date created: 2021-10-13T09:50:20Z

• owner: rgildein

• assignee: rgildein

• the launchpad url

[esunar]

(by rgildein) PR: https://github.com/canonical/juju-verify/pull/38