Include output power for EU863-870 for special RX2 frequency

[akarapatis]

Summary

Mention in the EU863-870 Summary that RX2 at frequency 869.525 has output power of 27dBm as implemented in https://github.com/TheThingsNetwork/lorawan-stack/blob/a537fe10c08864f22ada56c5dffbe9fd770482a6/pkg/band/eu_863_870_rp2_v1_0_4.go#L67

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Why do we need this ?

To have clear overview what is used
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What is already there? What do you see now?

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What is missing? What do you want to see?

ERP Output power of 27 dBm
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How do you propose to document this?

Include somewhere that output power is 27 dBm for RX2 in EU863-870
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Can you do this yourself and submit a Pull Request?

Yes I can
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