document SHOW_OPTIONS better

[kcrisman]

See ​this SO question. Basically, we might as well have some examples showing you can easily globally change some show option, like figsize or frame or something.

    SHOW_OPTIONS = dict(filename=None,
                        # axes options
                        axes=None, axes_labels=None, axes_pad=.02,
                        base=None, scale=None,
                        xmin=None, xmax=None, ymin=None, ymax=None,
                        # Figure options
                        aspect_ratio=None, dpi=DEFAULT_DPI, fig_tight=True,
                        figsize=None, fontsize=None, frame=False,
                        title=None, title_pos=None, transparent=False,
                        # Grid options
                        gridlines=None, gridlinesstyle=None,
                        hgridlinesstyle=None, vgridlinesstyle=None,
                        # Legend options
                        legend_options={}, show_legend=None,
                        # Ticks options
                        ticks=None, tick_formatter=None, ticks_integer=False,
                        # Text options
                        typeset='default')

Like

sage.plot.graphics.Graphics.SHOW_OPTIONS['figsize']=2