Add text about sorting code units

[aphillips]

Quoting https://github.com/w3c/i18n-activity/issues/362 :

I think it would be useful to add something to specdev describing the aims and limitations of the sort they are proposing. We'd need to create a succinct do/don't sentence, then add a paragraph of explanation, which could include:

This matches the Array.prototype.sort on an Array of Strings. This ordering compares the 16-bit code units in each string, producing a highly efficient, consistent, and deterministic sort order. The resulting list will not match any particular alphabet or lexicographical order, particularly for code points represented by a surrogate pair.

[r12a]

[from Addison]

Section 5.7 talks about sorting and only included four requirements from Charmod-Fundamentals with no explanation. I added all of the text [3] plus one new requirement pretaining to internal (non-linguistic) sorting:

[r12a]

Ed nits: require a program-internal fast deterministic sorting -> require a program-internal, fast, and deterministic sorting

[aphillips]

Done.