Update section on union datatypes

[kleihan]

Section 3.7.1.4 Union datatypes for code sets does not explain the use case of this in the context of a standards organization using Orchestra. The current text is as follows.

Code sets may have a second datatype to extend the list of values defined as codes with the underlying datatype given by the type attribute. Orchestra supports this by means of the unionDataType attribute of the <codeSet> element.

An Orchestra file published by a firm exposes its service offering, so it would not allow users to define their own codes. The use case for this is the FIX standard staking out reserved ranges of code values to extend the standard.

Hence the limited number of datatypes that can be used for a union (ReservedXXXPlus, Qty, Tenor). The use case for a firm is to indicate to the counterparty whether code values not explicitly defined in the Orchestra XML file will be accepted. It is not about a union of two different datatypes but about permitting additional values of the same datatype.

This issue was initially proposed by @donmendelson.