Add clarification about case sensitivity

[hollasch]

While it is possible to properly perform case-insensitive comparison, the process is complicated and must be explicit in the face of multiple locales. We could specify that all identifiers (keys, element types, and other IDs) are transformed to lowercase in the English locale, or processed with a standard foldcase implementation, this becomes onerous when behavior must be identical across any number of implementations, software languages and libraries.

In the interest of keeping the specification simple and easy to implement, simply dictate that all identifiers are case-sensitive, and possibly advocate for a convention of using lowercase (perhaps).

Another option—though messy—would be to specify that all ASCII characters will be compared case-insensitive in the English locale. It's an exception that would get a lot of utility, but is also an ugly inconsistency.

[hollasch]

See https://www.w3.org/International/wiki/Case_folding