DOCS-35260: Document MapChangeSet for Flutter

[cbullinger]

Pull Request Info

Jira ticket: https://jira.mongodb.org/browse/DOCSP-35260

• React to Changes - Flutter SDK: Update page to document support for RealmMap change listeners

Reminder Checklist

Before merging your PR, make sure to check a few things.

• [x] Did you tag pages appropriately?
  • genre
  • meta.keywords
  • meta.description
• [x] Describe your PR's changes in the Release Notes section
• [x] Create a Jira ticket for related docs-app-services work, if any

Release Notes

• Flutter SDK
  • Realm/Realm Database/React to Changes: Add information on registering a change listener on a RealmMap. -->

Review Guidelines

REVIEWING.md

[github-actions[bot]]

Readability for Commit Hash: e2fb948b282f9f1f9674af895d67100cfaba026a

You can see any previous Readability scores (if they exist) by looking at the comment's history.

Readability scores for changed documents:

• source/sdk/flutter/realm-database/react-to-changes: Grade Level: 7.9, Reading Ease: 59.5

For Grade Level, aim for 8 or below.

For Reading Ease scores, aim for 60 or above:

Score	Difficulty
90-100	Very Easy
80-89	Easy
70-79	Fairly Easy
60-69	Medium
50-59	Fairly Hard
30-49	Hard
0-29	Very Hard

For help improving readability, try Hemingway App.

[docs-builder-bot]

✨ Staging URL: https://preview-mongodbmongodb.gatsbyjs.io/realm/master/

🪵 Logs

• job log: ddad3c1d5db0424071808c37c098757c1c7ba836