Document $blogname, $blog_title, $errors, $domain, $path global in validate_another_blog_signup()

[mukesh27]

global $blogname, $blog_title, $errors, $domain, $path are used but it is not mentioned in the documentation of the validate_another_blog_signup() function.

[jeremyfelt]

Thanks @mukesh27, this looks good. I'm going to change the wording just a bit in commit. Note for future documentation patches how the type, name, and description are aligned with spaces.

[jeremyfelt]

In 43326:

Docs: Document globals in validate_another_blog_signup().

Props mukesh27.
Fixes #43594.