DOC: Improve reindex docstring and examples

[Winterflower]

Documentation here http://pandas.pydata.org/pandas-docs/stable/generated/pandas.DataFrame.reindex.html
could use some improvement

• mention that ffill, bfill, method only available for monotonically increasing and decreasing indices
• add a ton of examples to help users navigate all the various options
• add note regarding reindex fails on non-unique index #2067

Pull req is in the works.