docs: clarify iOS call detection behavior with UITimer polling

[liannacasper]

Motivation

• Clarify that Display.isInCall() on iOS is driven by app lifecycle transitions and that polling it with a UITimer can miss the interruption window because timers are paused while the app is inactive, so developers should use lifecycle callbacks instead.

Description

• Updated docs/developer-guide/Miscellaneous-Features.asciidoc to add a note describing the UITimer polling limitation, explaining why the flag can be missed and recommending checking call interruption state from lifecycle callbacks such as stop()/start() rather than periodic polling.

Testing

• Documentation-only change; no automated tests were required or modified.

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Codex Task

[github-actions]

Developer Guide build artifacts are available for download from this workflow run:

• Developer Guide HTML package
• Developer Guide PDF
• AsciiDoc linter report
• Vale report
• Unused image report

Developer Guide quality checks:

• AsciiDoc linter: 1 issue(s) flagged (1 errors) (exit code 1) (report)
• Vale: 18579 alert(s) (2701 errors, 6343 warnings, 9535 suggestions) (exit code 1) (report)
• Image references: 33 unused image(s) found (report)

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• img/uwp-app-manager-apps.png
• img/uwp-certificate-generator.png
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• img/uwp-cn1settings-disk-icon.png
• img/uwp-cn1settings-uwp.png
• img/uwp-cn1settings-windows-settings-menu-item.png
• img/uwp-dashboard-download-appxbundle.png
• img/uwp-extract-zip-file.png
• img/uwp-mobile-recently-added.png
• img/uwp-mobile-settings-developer-mode.png
• ... and 23 more