Correct return values for update_metadata() and related functions.

fixes #21864.
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git-svn-id: http://core.svn.wordpress.org/trunk@27050 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Correct @since value. see #26869.
Built from https://develop.svn.wordpress.org/trunk@27192


git-svn-id: http://core.svn.wordpress.org/trunk@27051 1a063a9b-81f0-0310-95a4-ce76da25c4cd