Update "Button" documentation

[didoo]

📌 Summary

The information that the tertiary button needs always an icon was buried in the examples and some people missed this critical information. Better to move it to the context of the icon prop.

🛠️ Detailed description

In this PR I have:

• moved the information in a more prominent/contextual position.

📸 Screenshots

How it looks now:

---

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[amyrlam]