PHPDoc correction for WP_Theme::get

[david.binda]

The PHPDoc for the WP_Theme::get function says it returns string|false (added in r32547 ), but it actually may return an array for "Tags" header (via WP_Theme::sanitize_header).

mixed, as used in WP_Theme::sanitize_header may better document the actual state.

Related code:

• https://core.trac.wordpress.org/browser/trunk/src/wp-includes/class-wp-theme.php?rev=45932#L849
• https://core.trac.wordpress.org/browser/trunk/src/wp-includes/class-wp-theme.php?rev=45932#L757
• https://core.trac.wordpress.org/browser/trunk/src/wp-includes/class-wp-theme.php?rev=45932#L761

[davidbaumwald]

Although doc changes can be made between beta 1 and RC, there are 200 bug tickets still to triage. With 5.3 Beta 1 landing in a couple of hours, this is being moved for consideration in 5.4 If a committer feels this change should be made before RC, they can move it back and commit.

[SergeyBiryukov]

In 46751:

Docs: Clarify that WP_Theme::get(), ::display(), and several related methods treat the Tags theme header as an array.

Props david.binda.
Fixes #48005.