docs: re-add `msg` to v4 -> v5 migration docs

[jgarplind]

Description

Was slightly confused to see msg was no longer in the macro API.

Sections below imply it still is.

Re-adding it here for completeness.

Types of changes

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Documentation update
• [ ] Examples update

Fixes # (issue)

Checklist

• [x] I have read the CONTRIBUTING and CODE_OF_CONDUCT docs
• [ ] I have added tests that prove my fix is effective or that my feature works
• [x] I have added the necessary documentation (if appropriate)

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[github-actions[bot]]

size-limit report 📦

Path	Size
./packages/core/dist/index.mjs	3.09 KB (0%)
./packages/detect-locale/dist/index.mjs	723 B (0%)
./packages/react/dist/index.mjs	1.65 KB (0%)
./packages/remote-loader/dist/index.mjs	7.26 KB (0%)