Document `status:success`, `status:failure`, and `status:pending`

[jsoref]

Code of Conduct

• I have read and agree to the GitHub Docs project's Code of Conduct

What article on docs.github.com is affected?

https://docs.github.com/en/issues/tracking-your-work-with-issues/using-issues/filtering-and-searching-issues-and-pull-requests

What part(s) of the article would you like to see updated?

https://docs.github.com/en/issues/tracking-your-work-with-issues/using-issues/filtering-and-searching-issues-and-pull-requests#about-search-terms

Add status:success, status:failure, and status:pending

Additional information

• Per https://github.blog/news-insights/filter-pull-requests-by-status/, this was added by the end of June 2015 but never documented (I'd tag the author, but their bio says "All of my popular repositories are bad ideas but I had fun making them! I used to work on this website but I don't anymore.").
• This was mentioned in a private discussion forum. I don't generally cross link to those, but if you ask me and if searches work I might be able to find the link.

[Sharra-writes]

@jsoref Yeah, easy test and definitely works. Should be an easy update, too, so I'll put a help wanted label on it and open it up to anyone who wants to add those search filters. Thanks!