doc: add hint for feature flag in wifi examples

[akesser]

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Submission Checklist 📝

• I have updated existing examples or added new ones (if applicable).
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Pull Request Details 📖

Description

I added a comment to the wifi examples that one may need to add the phy-enable-us feature

#2364

Testing

[bjoernQ]

Mid-term/long-term we will make this a default option but we most probably will also rework examples in general at some point in time

[MabezDev]

Thanks!