Correct WPF Popup article: remove Expander and tighten popup-implementing controls section

[Copilot]

The WPF Popup article listed Expander as a control that implements a popup, which is incorrect. This update also restructures the section text so each listed control directly explains how it uses a popup without redundant API repetition.

• Scope

  • Updated dotnet-desktop-guide/wpf/controls/popup.md in Controls that implement a Popup.

• Content corrections

  • Removed Expander from the popup-implementing controls list.
  • Kept the list to actual popup-backed controls: ToolTip, ContextMenu, and ComboBox.

• Section clarity improvements

  • Rewrote each bullet to:
    • Start with a bold xref.
    • Follow with a single sentence describing popup behavior for that control.
    • Keep the article link for further reading.
  • Removed duplicate xref mentions within bullet descriptions.

- **<xref:System.Windows.Controls.ContextMenu>**. Uses a popup to show command options for an element, usually when users right-click. For more information, see [ContextMenu](contextmenu.md).

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Internal previews

📄 File	🔗 Preview link
dotnet-desktop-guide/wpf/controls/popup.md	dotnet-desktop-guide/wpf/controls/popup

[Copilot]

Pull request overview

This pull request corrects the WPF Popup documentation by removing an incorrectly listed control and tightening the “Controls that implement a Popup” section so each listed control clearly describes its popup behavior.

Changes:

• Removed Expander from the list of controls that implement a popup.
• Rewrote the remaining bullets (ToolTip, ContextMenu, and ComboBox) to describe how each uses a popup while avoiding repeated API mentions.
• Added/standardized “For more information” links for each listed control.