[css-grid] fr-unit section clarification

Hi,

I think we should add some clarification in this section:
https://drafts.csswg.org/css-grid/#fr-unit

If you read this:
"Each column or row’s share of the free space can be computed as the
column or row’s <flex> * <free space> / <sum of all flex factors>."

And you've just one track with "0.5fr" in a 800px width grid container.
You might consider that the result should be:
	0.5 * 800 / 0.5 = 800

But this is wrong, if you go to the section:
https://drafts.csswg.org/css-grid/#algo-find-fr-size

Where you can read this:
"Let flex factor sum be the sum of the flex factors of the flexible
tracks. If this value is less than 1, set it to 1 instead."

So the good result is:
	0.5 * 800 / 1 = 400

I think it'd be nice to explain it properly in the "fr-unit section" or
at least link to "flex factor sum" from there.

Bye,
  Rego