Docs: Add a missing notation for the `$args` parameter in the DocBlock for...

Docs: Add a missing notation for the `$args` parameter in the DocBlock for `wp_xmlrpc_server::_multisite_getUsersBlogs()`.

Also adds a missing `@since` version. The method was introduced in [12852].

See #32246.

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git-svn-id: https://core.svn.wordpress.org/trunk@35921 1a063a9b-81f0-0310-95a4-ce76da25c4cd