Documentation exposes internal expression

[dgaubert]

Category Index is an internal expression but is documented as a public function but it's not.

const viz = new carto.Viz(`
      @duration: 30
      @animation: animation(linear($year, 1662, 1855), @duration, fade(0.1, ${Number.MAX_SAFE_INTEGER}))
      strokeWidth: 0
      color: ramp(categoryIndex($nationality), PRISM)
    `);

=>

Uncaught CartoParsingError: [Error] Invalid function name 'categoryIndex'.
    at parseFunctionCall (https://libs.cartocdn.com/carto-vl/v1.2/carto-vl.min.js:6:399501)
    at parseNode (https://libs.cartocdn.com/carto-vl/v1.2/carto-vl.min.js:6:399561)
    at https://libs.cartocdn.com/carto-vl/v1.2/carto-vl.min.js:6:399441
    at Array.map (<anonymous>)
    at parseFunctionCall (https://libs.cartocdn.com/carto-vl/v1.2/carto-vl.min.js:6:399418)
    at parseNode (https://libs.cartocdn.com/carto-vl/v1.2/carto-vl.min.js:6:399561)
    at parseVizNamedExpr (https://libs.cartocdn.com/carto-vl/v1.2/carto-vl.min.js:6:399140)
    at https://libs.cartocdn.com/carto-vl/v1.2/carto-vl.min.js:6:409517
    at Array.map (<anonymous>)
    at parseVizDefinition (https://libs.cartocdn.com/carto-vl/v1.2/carto-vl.min.js:6:409494)