Adds content to the header of the admin theme.
This hook is fired by the
header.php script, which is loaded by a call to
head() while it is populating the
<head> tag for the admin theme.
Functions attaching to this hook can directly output HTML into the
<head>, or use functions like
Plugins will likely not need to add content to the admin-side
<head> for every page, but the passed request object can be used to include content on particular pages only.