DOC: slice_indexer correction + examples
The stated return value of ``Index.slice_indexer`` in the doc string is wrong, the method can only return slices. Also added some examples.

In addition, some very minor corrections on the user guide text on ``IntervalIndex`` (moved the versionadded to the chapter top +  a spelling correction).  EDIT: These are not related to the ``.slice_indexer`` changes, but are attached to this PR as the changes are very small.