Add missing documentation for the `xmlrpc_element_limit` hook in wp-includes/class-IXR.php.

Props ankitgadertcampcom, kpdesign.
Fixes #29977.

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git-svn-id: http://core.svn.wordpress.org/trunk@30734 1a063a9b-81f0-0310-95a4-ce76da25c4cd