add bins argument to Histogram function
The primary argument used when plotting histograms is `bins`. This argument is not in the pandas documentation so you do not see it when auto-tabbing for documentation. I have added  `bins=10` as an argument to the dataframe and series histogram function while also adding a line of documentation. `10` is the matplotlib default bin value so this is only making `bins` visible, not adding or changing any plotting functionality.

can you move the argument to the end of the arg list pls.
 I'll look closer at this later, but the usual convention for our plotting functions is that any keyword arguments used in matplotlib's `plot` or `hist` or `bar`, etc. are passed in `**kwargs`.

Also you may want to use `df.plot(kind='hist')`.
 @jreback updated argument list
@TomAugspurger  I think this is import documentation for histograms. You can use `df.plot(kind="hist")` but I typically use `series.hist(bins=20)`.  For historical reasons or otherwise dataframes and series can call `hist` directly so having histogram-specific information there makes sense to me. I can certainly see why you wouldn't want `bins` in `df.plot()`. 
 I misspoke earlier. `hist` isn't even a supported kind for `df.plot()`
 @TomAugspurger what should we do with this?
 I'm happy whatever you decide to do. I just rebased against upstream/master in case its a go.
 The downside is that this will be inconsistent with all our other plotting functions, where the arguments that go directly to the maptlotlib function are passed in the **kwargs section. But, it's good for discoverability, so let's merge.

@zachcp could you move the `bins` down in the docstring to match the order in the function signature?

Looks like you had a test fail too.
 made a couple minor fixes to padd test (need to pass bins as arg) and added two small tests.
 OK thanks.

Don't need a release notes entry since this is really just a doc change. There isn't any change to the API
 Oops, looks like we missed a call to hist in `hist_frame`. Not surprising given how spread out the code is. I've got a PR coming
 merged https://github.com/pydata/pandas/pull/6935
