awk: Merge upstream manpage updates

Merge the upstream manpage upades into awk.1. This goes through upstream
hash 9acc510. Upstream man page is written in raw nroff with "an"
macros, rather than in mandoc, so convert to mandoc as well. The man
page isn't updated on imports automatically, plus our man page has
diverged somewhat from upstraem's so it's not a mechanical change...

PR: 230730

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