Better descriptions for Polar Charts keys

- improve desc. for 'r', 't' in Scatter, Bar & Area
- improve desc. for 'direction' and 'orientation' in Layout
- delete 'defaultcolorrange', 'opacity' and 'needsEndSpacing' in Layout
- add details to desc. of 'range' in AngularAxis
- 'domain' in AngualarAxis added back (but does not work)
- showline now generated with make_showline()
- add 'number_array' val type (for 'r')
- 'r' and 't'  are listed last in Bar (along with other artifact keys)

And a few small fixes

- better handling of 'required' for conditionally required keys 
- correct 'orientation' val type (now "'v' | 'h'")
- fix typo in 'scl' example (now 'Greys')
- fix typo in 'domain' example (now [0,0.4], [0.6,1])
for now, we support 

```html
<sup>
<sub>
<b>
<i>
<a>
<span>
<br>
<em>
```