DOC: Improve reindex docstring and examples
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 https://github.com/pydata/pandas/issues/2067

Pull req is in the works.
