DOC/VIS: plotting on current figure/axis with Series/DataFrame.plot()
From [SO](http://stackoverflow.com/questions/26845727/pandas-plot-multiple-columns-of-a-timeseries-with-labels-example-from-pandas-d/26845829#26845829), I noticed that `Series.plot()` plots on the current axis if no ax is specified, and `DataFrame.plot` not (so creates a new figure when no ax is specified).

Some things about this:
- Is this deliberate? I think this is possible:
  - [doc](http://pandas.pydata.org/pandas-docs/stable/generated/pandas.Series.plot.html) of `Series.plot()` mention this on `ax`: "If not passed, uses gca()", while the docs for `DataFrame.plot()` don't say this
  - The idea is maybe that a series you often want to add to a figure, and a DataFrame (with already multiple columns) you just want to plot?
- But still, this is somewhat inconsistent?

So, if this is deliberate, we should:
- clarify the docstring of `DataFrame.plot()` (now nothing is said about this)
- remove the `plt.figure()` calls from the documentation, as these have no effect (eg second plot here: http://pandas.pydata.org/pandas-docs/stable/visualization.html#basic-plotting-plot). This is what caused the confusion on SO.

or if not:
- implement that `DataFrame.plot()` also first tries to get the current axis with `gca`

@TomAugspurger @sinhrks
