EXPRESS_ — Define SpecializedType

In 8.6.11 (SpecializedType), the Definition was somehow garbled by the Note.

Following the pattern in 8.6.6 and 8.6.10, the Definition should be something like: a DefinedType representing an EXPRESS defined data type whose underlying_type is not a SELECT type nor an ENUMERATION.

Alternatively, we could actually provide definitions for these three classes that give the reader some understanding of the intent.

Repair the definition of SpecializedType in 8.6.11, and the Note, which refers to the wrong clauses of ISO 10303-11. Expand the definitions in 8.6.6 and 8.6.10 to give some indication of the meaning of the types.