chore: clarify frozen vs allow_mutation in docstring

[themattmorris]

Change Summary

Clarifies that frozen is the inverse of v1.allow_mutation in config docs.

Related issue number

fix #8488

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist - NA
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

please review

[codspeed-hq]

CodSpeed Performance Report

Merging #8495 will not alter performance

_{Comparing themattmorris:frozen-vs-allow-mutation-docstring (90c8f85) with main (53bb2a8)}

Summary

✅ 10 untouched benchmarks

[sydney-runkle]

Looks great, thanks!