docs: improve documentation for merge behavior

[schmidt-sebastian]

Addresses firebase/firebase-admin-node#987

[changeset-bot]

⚠️ No Changeset found

Latest commit: 67afa76

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[google-oss-bot]

Binary Size Report

Affected SDKs

• firebase

  Type	Base (9295bb0)	Head (dc3e108)	Diff
  firebase-firestore-lite.js	245 kB	245 kB	+76 B (+0.0%)

Test Logs

• Base (9295bb0b): https://github.com/firebase/firebase-js-sdk/actions/runs/1603106571
• Head (dc3e1081): https://github.com/firebase/firebase-js-sdk/actions/runs/1612706751

[google-oss-bot]

Size Analysis Report

Affected Products

No changes between base commit (9295bb0) and head commit (dc3e108).

Test Logs

• Base (9295bb0): https://github.com/firebase/firebase-js-sdk/actions/runs/1603106571
• Head (dc3e108): https://github.com/firebase/firebase-js-sdk/actions/runs/1612706751