Document the ordinal case better

[zbraniecki]

During TC39 presentation we once again had to explain the value of type: ordinal.

I think it would be worth better documenting the use cases.
I wrote some explanation here: tc39/ecma402#34 (comment)

We should turn it into part of the spec docs.

[caridy]

Welsh is one of the examples of a language that requires different forms or pluralization for ordinal values, here are few examples:

http://www.unicode.org/cldr/charts/29/supplemental/language_plural_rules.html#cy

n=3, which selects "few", can be used to produce: "3 cath"

while

n=6, which selects "many", can be used to produce "6 chath"

As you can see in the example above, the rest of the sentence defer based on the pluralization rule.