DOC: Fix docstring description of zoom-history keyboard shortcuts.

A few of the examples stated that you could scroll through the zoom history using alt-left-arrow and alt-right-arrow. The `ZoomTool`, however, responds to `control-z` and `control-y`.

Also, one of the examples turned off zoom-box selection (by setting `drag_button=None`). Since the docstring refers to the zoom box and zoom history, this limitation was also removed.

Closes #15