Docs: Add a missing summary, `@since` version, and parameter descriptions to...

Docs: Add a missing summary, `@since` version, and parameter descriptions to the DocBlock for `wp_xmlrpc_server::add_enclosure_if_new()`.

Introduced in [10383].

See #32246.

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git-svn-id: https://core.svn.wordpress.org/trunk@35928 1a063a9b-81f0-0310-95a4-ce76da25c4cd