[css-colors-5] Minor syntax problems

<color> is missing <color-mix()>.
<cmyk-component> is missing none (also applies to the <alpha-value> of device-cmyk()).

All color functions (with the exception of those using the legacy color syntax) allow any of their components to be specified as none.

<custom-params> should define a list of whitespace (vs. comma) separated parameters (+ instead of #).
<rectangular-color-space> is defined in CSS Color 4 with a wider range of values than in CSS Color 5.

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