Twenty Fifteen: correct number of menus used in comments

Props jeherve, see #30149


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Twenty Fifteen: docs correction and cleanup.

Props DrewAPicture, see #30149.


Built from https://develop.svn.wordpress.org/trunk@30569


git-svn-id: http://core.svn.wordpress.org/trunk@30559 1a063a9b-81f0-0310-95a4-ce76da25c4cd