Revert "Remove "Keep Sidebar Open" setting."

This reverts commit 56b996be72d5e296e38f5358919275dc08199b05.

This is probably one of those settings that gets changed once then left alone,
so something that half works beats nothing at all.
Clarify "Keep Sidebar Open" explanation on iOS 5.

Best I could figure out with UISplitViewController.
"Keep Sidebar Open" defaults to "Landscape".