Clarify the docs and add more unit tests for wp_set_object_terms().

props DrewAPicture.
fixes #26570.
Built from https://develop.svn.wordpress.org/trunk@28951


git-svn-id: http://core.svn.wordpress.org/trunk@28748 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Fix parameter description for `$append` in `wp_set_object_terms()` inline docs.

See #26570.

Built from https://develop.svn.wordpress.org/trunk@28952


git-svn-id: http://core.svn.wordpress.org/trunk@28749 1a063a9b-81f0-0310-95a4-ce76da25c4cd