United States Core Data for Interoperability (USCDI) requires a reaction

it was missing from the initial draft ONC documentation and thus not addressed when drafting the updates to US Core

It is defined in the final documentation here: https://www.healthit.gov/isa/sites/isa/files/2020-03/USCDI-Version1-2020-Final-Standard.pdf

propose adding:

• AllergyIntolerance.reaction 0..* MS
• AllergyIntolerance.reaction.manifestation 1..* MS with an extensible binding to http://hl7.org/fhir/ValueSet/clinical-findings
• update narrative and examples to reflect changes