Improve bar documentation especially for color

[MarcSkovMadsen]

Addresses a users comment about limited bar documentation and no documentation of color. See https://discourse.holoviz.org/t/is-there-a-way-to-add-a-specific-color-to-each-group-in-a-grouped-bar-plot-like-hue-in-seaborns-barplot/5066

Also fixes a few docstring bugs identified when rendering the output of hvplot.help('bar') on Discourse.

hvplot-pandas-bar-speedup.mp4

Issues identified while working on this

The main issue is that lots of things does not work for Matplotlib and Plotly backends as they do for Bokeh backend

• Options for legend_position incorrect in docs #1034
• legend not working for bar plot with Plotly backend #1035
• multiindex not working for bar plot with Plotly backend #1036
• color not working for bar plot with Plotly backend #1037
• Categories written on top of each other for multiindex bar plot with Matplotlib backend #1038
• AttributeError when using name of column as color argument for bar plot with Matplotlib backend #1039

[maximlt]

Thanks a lot!