[wp-trac] [WordPress Trac] #41619: Fix documentation issues for `domain_exists()`

[WordPress Trac]

#41619: Fix documentation issues for `domain_exists()`
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 Reporter:  flixos90            |      Owner:
     Type:  defect (bug)        |     Status:  new
 Priority:  normal              |  Milestone:  4.9
Component:  Networks and Sites  |    Version:
 Severity:  normal              |   Keywords:  needs-patch good-first-bug
  Focuses:  docs, multisite     |
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 There are several issues with the documentation for `domain_exists()` and
 its filter:

 * It's actually the domain-path combination that is checked. So the term
 "blogname" should probably be replaced with something like "domain-path
 combination".
 * In the filter documentation the term "site ID" should be used instead of
 "blog_id".
 * There's no description for the return type, and it's incorrectly
 highlighted as `int` where it should be `int|null`.
 * The `$wpdb` reference in the docblock can be removed as it's actually
 not used in the function.

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Ticket URL: <https://core.trac.wordpress.org/ticket/41619>
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