searching: any assignee; excluding missing metadata

[jrchamp]

• Add "any assignee" example
• Document that "missing metadata" qualifiers cannot be combined with the exclusion/minus symbol

Why:

Had a team meeting and wanted to get all of the assigned issues on the screen at once. The documentation said it was impossible, but it is possible and I proved it. Initially, I tried to negate the no:assignee qualifier, but - contrary to the documentation - that did not work.

Closes: #31663 Closes: #31664

What's being changed (if available, include any code snippets, screenshots, or gifs):

Tips, Assignee and Missing Metadata sections now better reflect reality.

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The table contains an overview of files in the content directory that have been changed in this pull request. It's provided to make it easy to review your changes on the staging site. Please note that changes to the data directory will not show up in this table.

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Content directory changes

You may find it useful to copy this table into the pull request summary. There you can edit it to share links to important articles or changes and to give a high-level overview of how the changes in your pull request support the overall goals of the pull request.

Source	Preview	Production	What Changed
search-github/searching-on-github/searching-issues-and-pull-requests.md	fpt ghec ghes@ 3.12 3.11 3.10 3.9	fpt ghec ghes@ 3.12 3.11 3.10 3.9

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fpt: Free, Pro, Team ghec: GitHub Enterprise Cloud ghes: GitHub Enterprise Server

[nguyenalex836]

@jrchamp Thanks so much for opening a PR! I'll get this triaged for review ✨

[nguyenalex836]

@jrchamp Thank you for this wonderful addition! Tested myself, and this is certainly a handy trick ✨ I'll update the branch and get this merged once tests are passing 🍏

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