4.3.2: Field descriptor cleanups

A few minor cleanups to improve the presentation of section 4.3.2:

- Rename "ObjectType" to "ClassType" (distinguished by using a class (or interface) name)

- State as a general rule in 4.3.1 that fixed-width characters in the grammar represent ASCII characters

- Opening sentence should mention where field descriptors can be used

- Table 4.3-A should cross-reference Chapter 2 for the meaning of types, rather than trying to restate it here in the "Interpretation" column