docs(firestore): add an example to the detailed comment in compareUtf8Strings()

[dconeybe]

This PR enhances the documentation for the compareUtf8Strings() method by adding a concrete example. This example illustrates the nuances of comparing Unicode strings, particularly how characters represented by surrogate pairs in UTF-16 can compare differently than the lexicographical comparison of their UTF-8 encoding.

This PR is in response to review comment firebase/firebase-android-sdk#7098 (review) and is a port of firebase/firebase-android-sdk#7113

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Latest commit: bb4653d

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[google-oss-bot]

Size Report ¹

Affected Products

No changes between base commit (a029ce3) and merge commit (658bd74).

Test Logs

• Base (a029ce3): https://github.com/firebase/firebase-js-sdk/actions/runs/16131357566
• Merge (658bd74): https://github.com/firebase/firebase-js-sdk/actions/runs/16131442606

1. https://storage.googleapis.com/firebase-sdk-metric-reports/hTIQsNbWEn.html

[google-oss-bot]

Size Analysis Report ¹

Affected Products

No changes between base commit (a029ce3) and merge commit (658bd74).

Test Logs

• Base (a029ce3): https://github.com/firebase/firebase-js-sdk/actions/runs/16131357566
• Merge (658bd74): https://github.com/firebase/firebase-js-sdk/actions/runs/16131442606

1. https://storage.googleapis.com/firebase-sdk-metric-reports/5MrwQmzB4f.html