docs: add caveats for Electron PipeWire implementation

[byquanton]

Description of Change

The Pipewire Capturer implementation in Electron has some Caveats, but those are only documented in the codebase.

electron/shell/browser/api/electron_api_desktop_capturer.cc

Line 253 in fed7304

// PipeWire returns a single source, so index is not relevant.

electron/shell/browser/api/electron_api_desktop_capturer.cc

Lines 285 to 286 in fed7304

	// Some capturers like PipeWire support a single capturer for both screens
	// and windows. Use it if possible, treating both as window capture

Checklist

• relevant API documentation, tutorials, and examples are updated and follow the documentation style guide

Notes: none

[welcome]

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[byquanton]

What is required to get this merged?

[erickzhao]

docs: lgtm

[electron-docs-reviewer]

Approving on behalf of the Electron Docs Team

[electron-release-clerk]

No Release Notes

[trop]

I have automatically backported this PR to "39-x-y", please check out #48713