DOC: Add infer frequency to docs
Was there a reason infer_freq and inferred_freq weren't in the docs?  Did I pick the right spot to add it? Does the feature work well?

I set out looking for a feature like this, searching "guess frequency", "auto detect frequency".  

minor change, ping when pushed.
 @jnmclarty Could you also update the `infer_freq` docstring with:
- it also works for TimedeltaIndex
- it raises a ValueError when not having 3 values? (now it only mentions a TypeError when the the provided values are no DatetimeIndex)
 @jorisvandenbossche side issue, we don't appear to have `PeriodIndex` at all in `api.rst`....do we have an issue for this?
 Ideally, we should also resolve this one: https://github.com/pydata/pandas/issues/5082
 On the PeriodIndex API docs: yes, the issue is here: https://github.com/pydata/pandas/issues/6384
 @jorisvandenbossche on #5082 

In reality, I just think we could just fix `infer_freq` & `infered_freq` to return an actual offset object (which reprs to a string). rather than adding addtional methods. These compare correctly to strings anyhow.
 @jreback Okay to merge.
 thanks!
