Better descriptions for Polar Charts keys

- improve desc. for 'r', 't' in Scatter, Bar & Area
- improve desc. for 'direction' and 'orientation' 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')

And a few small fixes

- required_when is not a function anymore (not in val_types)
- 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])