update `vertextcolor` description to note the range of the values

[chriddyp]

I was just reading through a community member's notebook (https://plot.ly/~empet/15040/plotly-mesh3d-from-a-wavefront-obj-f/#/) and came across this comment:

Indeed, looks like the vertexcolor description is a little terse, could mention that this color needs to be normalized between 0 and 1, unlike other color mappings:

plotly.js/src/traces/mesh3d/attributes.js

Lines 150 to 158 in 83fd651

	vertexcolor: {
	valType: 'data_array',
	role: 'style',
	editType: 'calc',
	description: [
	'Sets the color of each vertex',
	'Overrides *color*.'
	].join(' ')
	},

[etpinard]

Hmm. RGB colors seem to work ok:

https://codepen.io/etpinard/pen/GPNwYX?editors=0010

[archmoj]

@etpinard I think the point is that when users have 3-D models in OBJ format (where the alpha is written in the range of 0 to 255), they need to map it to fit in [0,1]. So they can't use their raw alpha values such as 127 in this example.