Fixes the documentation and examples of the append methods

[darnuria]

Now documentation of KeyExist Error describe the case of append with unordered data.

Also change the examples to use the append function.

Related issue

Fixes #163

PR checklist

Please check if your PR fulfills the following requirements:

• [x] Does this PR fix an existing issue, or have you listed the changes applied in the PR description (and why they are needed)?
• [X] Have you read the contributing guidelines?
• [x] Have you made sure that the title is accurate and descriptive of the changes?

[Kerollmops]

Thank you very much ❤️

[darnuria]

Thanks for the review and your time!