Make sure widget at index=0 is removed from sidebar when deactivated

Without this change, deactivating a widget at the beginning of a
widget area will appear twice, once in the original sidebar and again
in the Inactive Widgets sidebar.
Update move_sidebar_widget DocBlock

Indicate that it also moves to another sidebar
Merge pull request #1202 from x-team/widget-deactivate-fix

Make sure widget at index=0 is removed from sidebar when deactivated