Update register_taxonomy_for_object_type() phpdoc to reflect that the...

Update register_taxonomy_for_object_type() phpdoc to reflect that the post_type must be a string only, no arrays allowed. Props transom. fixes #19795

git-svn-id: https://core.svn.wordpress.org/trunk@19730 1a063a9b-81f0-0310-95a4-ce76da25c4cd