Document special 'alt' inheritance

The @alt attribute has special inheritance behavior. If you follow the normal inheritance chain and find nothing down in root, then you start the search at the original locale but removing the @alt attribute.

So it behaves as if there were an alias element down in root for every element that takes an @alt attribute.

This is not clearly enough stated in the spec, and should also be pointed to from the following sections:

• https://unicode-org.github.io/cldr/ldml/tr35.html#Multiple_Inheritance

• https://unicode-org.github.io/cldr/ldml/tr35.html#Alias_Elements