Define explicit concepts in ABNF for referenced sets of rules

There are a few places today where we reference things like "xxx" rule in the ABNF. That is, a set of rules that match a particular pattern.

For example, [CSDL] contains the following text:

"Default values MUST be represented according to the xxxValue rule defined in [OData-ABNF] that is appropriate for the type of the property."

Which is intended to mean the "unprefixed primitive literal values". However, users looking at the documentation won't find "xxxValue" and, in fact, won't find "xxxValue" rules for some of the primitives (for example, there is no "StringValue".

Also note that constant expressions reference the xml schema serialization rules, rather than the rules in our ABNF (which are hopefully equivalent?). To make sure we are in sync, documents should reference the primitive value serialization defined in the [ABNF], which should reference xml schema as appropriate.