Inline documentation cleanup for 4.0 audit.

Document the first parameter, `$time`, in the 'oembed_ttl' filter, added in [28972].

See #14759 and #28885.

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git-svn-id: http://core.svn.wordpress.org/trunk@28944 1a063a9b-81f0-0310-95a4-ce76da25c4cd