docs(material): clarify Shadow DOM limitations

[Gaalbu]

Clarify the styling and overlay-container limitations that remain when Angular Material components are used inside Shadow DOM.

Closes #13837

Validation:

• git diff --check
• aislop scan --changes --json (100/100)

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[Gaalbu]

Closing this contribution because the repository requires the Google CLA and the CLA check cannot pass for this account. The change will not be pursued here.