DOC: write docs for the ``ax`` keyword

[jakevdp]

This is a common pattern with pandas plots:

ax = df.plot.line()
df.plot.area(ax=ax)

We could do a similar thing by adding an ax argument to methods and using vega-lite layering internally... but corner cases may get a bit complicated.

[jakevdp]

done in 0936277; leaving this issue open to remind me to add it to the docs.