Update expander.md

[AndreiMisiukevich]

Docs for Toolkit PR #1349

What changes to the docs does this PR provide?

Add description for three new properties in Expander control

PR Checklist

Please check if your PR fulfills the following requirements:

• For new pages, used the provided template.
• For new features, added an entry in the Table of Contents.
• Ran against a spell and grammar checker.
• Contains NO breaking changes.

[davidbritch]

Docs Build status updates of commit 8c65f34:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/views/expander.md	✅Succeeded	View

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

For any questions, please:

• Try searching the docs.microsoft.com contributor guides
• Post your question in the Docs support channel

[jfversluis]

Change for Toolkit isn't merged yet. I think we will soon and then also release 1.2 soon after, let's sit on this for a bit until then

But thanks for adding this right away Andrei!

[AndreiMisiukevich]

@davidbritch @jfversluis can be merged now

[davidbritch]

Docs Build status updates of commit 5822e56:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/views/expander.md	✅Succeeded	View

For more details, please refer to the build report.

Note: Broken links written as relative paths are included in the above build report. For broken links written as absolute paths or external URLs, see the broken link report.

For any questions, please:

• Try searching the docs.microsoft.com contributor guides
• Post your question in the Docs support channel