WPDB Unit Tests: Fix a fatal error that occurs in the `test_strip_invalid_text()` test if the `mbstring` extension isn't activated in PHP.

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git-svn-id: http://core.svn.wordpress.org/trunk@30621 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Document the `$user_id` parameter added to the 'admin_color_scheme_picker' hook in 3.8.1.

Props Ipstenu.
Fixes #30551.

Built from https://develop.svn.wordpress.org/trunk@30632


git-svn-id: http://core.svn.wordpress.org/trunk@30622 1a063a9b-81f0-0310-95a4-ce76da25c4cd