Escape title tag in 'admin_title' hook docs

[SergeyBiryukov]

See ​https://developer.wordpress.org/reference/hooks/admin_title/, which only displays half of the page due to the lack of escaping:

<p>Filter the <title> content for an admin page.</p>

Apparently we need to escape <title> with backticks, like we do elsewhere, e.g. in wp_get_archives()​.

[DrewAPicture]

In 30502:

Convert <title> to "title tag" in the DocBlock summary for the admin_title hook.

Fixes #30437.