Twenty Ten: add helpful descriptions to sidebars. Fixes #15401.

git-svn-id: http://core.svn.wordpress.org/trunk@23477 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Twenty Ten: prevent overflow for large, captioned images. Fixes #22820, props obenland.

git-svn-id: http://core.svn.wordpress.org/trunk@23478 1a063a9b-81f0-0310-95a4-ce76da25c4cd