docs: use "id" instead of "label" for positions

[cedrikewers]

Description of Change

The documentation for the Menu and MenuItem classes describes the constructor options before, after, beforeGroupContaining and afterGroupContaining wrongly. It is said, that the placement would be relative to the MenuItem containing the specified labels. The placement however is relative to a MenuItem with the specified id as shown in the example in the Menu classes documentation. This PR changes the descriptions to reflect this.

Checklist

• PR description included and stakeholders cc'd
• relevant documentation, tutorials, templates and examples are changed or added
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: none

[welcome]

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[erickzhao]

docs: lgtm

[electron-docs-reviewer]

Approving on behalf of the Electron Docs Team

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[release-clerk]

No Release Notes

[trop]

I have automatically backported this PR to "30-x-y", please check out #41859