Fix documentation issues for `domain_exists()`

[flixos90]

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.

[flixos90]

Thanks for the patch @stevenlinx, looks good!

@DrewAPicture @jeremyfelt Any objections for the term "domain-path combination"? Is there a better one we should use?

[DrewAPicture]

Hmm. I think introducing the new terminology actually confuses it a bit more. You're correct that domain_exists() literally checks the domain path combination, though "domain-path combination" doesn't really convey what's being checked the way blogname sort-of does currently.

Rather than introduce new vernacular, I wonder if simply providing context for what "blogname" represents in the DocBlock description wouldn't be more consistent with the rest of core. For instance, in the newblogname hook doc, the explanation of what the [blog] name represents is pretty clear:

 * Filters the new site name during registration.
 *
 * The name is the site's subdomain or the site's subdirectory
 * path depending on the network settings.
 *

[flixos90]

@DrewAPicture That makes sense. Let's go with "site name" then (replacing the old terminology "blogname") and provide an explanation like the one you shared.

[flixos90]

Changes look good @stevenlinx, thanks for continuing to work on it!

@DrewAPicture You're good with this?

[DrewAPicture]

@flixos90 Looks good. All yours :-)

[flixos90]

In 41596:

Multisite: Fix documentation issues for domain_exists().

Props stevenlinx.
Fixes #41619.