DOC: add Raises, Examples and See Also sections to methods at_time/between_time/first/last
Added note that methods ``at_time``/``between_time``/``first``/``last`` requires that the index is a DateTimeIndex.

Added functioning examples to the method doc strings.

Added See Also section to the mentioned method doc string.

EDIT: The doc strings for ``DatetimeIndex.indexer_at_time`` and ``DatetimeIndex.indexer_between_time`` said they return a timeseries, while in reality they return arrays. I've corrected that, and made some minor updates to those doc strings in addition.