[Auth] Update docs for UserCredential

[sam-gc]

Related: #5712

[changeset-bot]

⚠️ No Changeset found

Latest commit: 946e70a

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[google-oss-bot]

Binary Size Report

Affected SDKs

No changes between base commit (88f2c70) and head commit (d422192).

Test Logs

• Base (88f2c706): https://github.com/firebase/firebase-js-sdk/actions/runs/1450534743
• Head (d422192c): https://github.com/firebase/firebase-js-sdk/actions/runs/1450643260

[google-oss-bot]

Size Analysis Report

Affected Products

No changes between base commit (88f2c70) and head commit (d422192).

Test Logs

• Base (88f2c70): https://github.com/firebase/firebase-js-sdk/actions/runs/1450534743
• Head (d422192): https://github.com/firebase/firebase-js-sdk/actions/runs/1450643260