Add screen reader text to announce that menu item accordion sections are expandable. Remove redundant title attribute.

props joedolson.
fixes #28866.
Built from https://develop.svn.wordpress.org/trunk@29239


git-svn-id: http://core.svn.wordpress.org/trunk@29023 1a063a9b-81f0-0310-95a4-ce76da25c4cd