doc: Document reusing payload (de)coders, note about TTIG migration

[nejraselimovic]

Summary

Ref: https://github.com/TheThingsIndustries/lorawan-stack-docs/issues/368

Changes

Added a note that the user needs to add one additional line to V2 payload decoder to make it fully compatible with TTS

Checklist

• [x] Scope: The referenced issue is addressed, there are no unrelated changes.
• [x] Run Locally: Verified that the docs build using make server, posted screenshots, verified external links.
• [x] Style Guidelines: Documentation obeys style guidelines in CONTRIBUTING.
• [x] Commits: Commit messages follow guidelines in CONTRIBUTING, there are no fixup commits left.