Twenty Thirteen: small tweaks to stylesheet description for clarity.

git-svn-id: http://core.svn.wordpress.org/trunk@24795 1a063a9b-81f0-0310-95a4-ce76da25c4cd
POT, generated from r24795

git-svn-id: http://core.svn.wordpress.org/trunk@24796 1a063a9b-81f0-0310-95a4-ce76da25c4cd