Fixed docblock in `cache_add` method in `WP_Theme` class

[itowhid06]

$data @param type of $data in cache_add method in WP_Theme class because in several instances array is being passed as $data

[williampatton]

Hey @itowhid06, thank you for the patch!

If you have looked to see that sometimes array is passed to this did you consider if it would be more useful to use string|array instead of mixed?

[itowhid06]

@williampatton, you are right. I've updated the patch(48096.1.diff).

[SergeyBiryukov]

In 46226:

Docs: Correct type for the $data parameter of WP_Theme::cache_add().

Props itowhid06.
Fixes #48096.