Documentation for externals in FHIRHelpers

[EvanMachusak]

The external functions in FHIRHelpers 4.0.1 (https://github.com/cqframework/clinical_quality_language/blob/master/Src/java/quick/src/main/resources/org/hl7/fhir/FHIRHelpers-4.0.1.cql) are not documented. Implementers need specifications on input and expected output.

Inline comments in the CQL would be good, but they also deserve a section in the CQL documentation (https://cql.hl7.org/index.html). FHIRHelpers is a required include when using some constructs in conjunction with the cql-to-elm tooling.