Some more tweaks for the documentation in wp-admin/includes/ms.php. see #25229.

git-svn-id: https://develop.svn.wordpress.org/trunk@25497 602fd350-edb4-49c9-b593-d223f7449a82
Clarify error handling and special cases in the `wp_insert_term()` phpdoc block.

props lgedeon, SergeyBiryukov. See #22801.


git-svn-id: https://develop.svn.wordpress.org/trunk@25498 602fd350-edb4-49c9-b593-d223f7449a82