[css-shapes-2] Minor problems with the new `<arc-command>`

[cdoublev]

Its production requires specifying the size with two <length-percentage>:

<arc-command> = arc [ 
     <command-end-point>
  && [ of <coordinate-pair> ]
  && <arc-sweep>? 
  && <arc-size>? 
  && [rotate <angle>]? 
]
<coordinate-pair> = <length-percentage>{2}

But then its contextual definition has a different value definition and a prose that allows omitting the second component value:

<arc-command> = arc [[<by-to> <coordinate-pair>] || [of <length-percentage>{1,2}] || <arc-sweep>? || <arc-size>?|| rotate <angle>? ]

[...] The of component specifies the size of the ellipse that the arc is taken from. The first <length-percentage> provides the horizontal radius of the ellipse and the second provides the vertical radius. [...]

If only one <length-percentage> is provided, both radiuses use the provided value. In that case, <percentage> values are resolved against the direction-agnostic size of the reference box (similar to the circle() function).

[smfr]

cc @tabatkins who did the most recent edits.