Firestore: Re-write API docs for COUNT API

[dconeybe]

The wording is copied from firebase/firebase-android-sdk#4143

[changeset-bot]

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Latest commit: b5af575

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[google-oss-bot]

Size Report ¹

Affected Products

• @firebase/firestore

  Type	Base (2d55fc6)	Merge (2235c77)	Diff
  browser	265 kB	265 kB	-40 B (-0.0%)
  esm5	329 kB	329 kB	-44 B (-0.0%)
  main	530 kB	530 kB	-99 B (-0.0%)
  module	265 kB	265 kB	-40 B (-0.0%)
  react-native	265 kB	265 kB	-40 B (-0.0%)

• @firebase/firestore-lite

  Type	Base (2d55fc6)	Merge (2235c77)	Diff
  browser	82.1 kB	82.1 kB	-40 B (-0.0%)
  esm5	98.3 kB	98.3 kB	-44 B (-0.0%)
  main	139 kB	139 kB	-99 B (-0.1%)
  module	82.1 kB	82.1 kB	-40 B (-0.0%)
  react-native	82.3 kB	82.3 kB	-40 B (-0.0%)

• firebase

  Type	Base (2d55fc6)	Merge (2235c77)	Diff
  firebase-firestore-lite.js	87.8 kB	87.7 kB	-40 B (-0.0%)
  firebase-firestore.js	313 kB	313 kB	-40 B (-0.0%)

Test Logs

• Base (2d55fc6): https://github.com/firebase/firebase-js-sdk/actions/runs/3137218969
• Merge (2235c77): https://github.com/firebase/firebase-js-sdk/actions/runs/3137643551

1. https://storage.googleapis.com/firebase-sdk-metric-reports/0TSfPZd23a.html

[google-oss-bot]

Size Analysis Report ¹

This report is too large (211,141 characters) to be displayed here in a GitHub comment. Please use the below link to see the full report on Google Cloud Storage.

Test Logs

• Base (2d55fc6): https://github.com/firebase/firebase-js-sdk/actions/runs/3137218969
• Merge (2235c77): https://github.com/firebase/firebase-js-sdk/actions/runs/3137643551

1. https://storage.googleapis.com/firebase-sdk-metric-reports/2G7CxiXqBR.html

[milaGGL]

LGTM