Add the definition of each code to Code subtype constants and ValueSet values

[lmsurpre]

Is your feature request related to a problem? Please describe.
Currently we generate Code SubTypes in the package com.ibm.fhir.model.type.code.
While working with values from this package, you need to understand what the allowed code values mean.
Currently, we add only the "display" value as javadoc which is typically almost the same as the constant's name. This is sufficient for some cases, but since the spec also includes a definition for each code, that would be good to add.

Describe the solution you'd like
Currently we have:

    /**
     * Invalid Content
     */
    public static final IssueType INVALID = IssueType.builder().value(ValueSet.INVALID).build();

What I'd like is:

    /**
     * Invalid Content
     *
     * <p>Content invalid against the specification or a profile.
     */
    public static final IssueType INVALID = IssueType.builder().value(ValueSet.INVALID).build();

Note: these classes under com.ibm.fhir.model.type.code are generated and so the "real" change would be to:

1. update fhir-tools to include the definition in the javadoc
2. regenerate the model
3. push all related changes (including the fhir-tools update)

I'd like the javadoc updated on both the constant AND the corresponding ValueSet enum.

source: https://www.hl7.org/fhir/valueset-issue-type.html

Describe alternatives you've considered
Just navigate to the corresponding spec page and read the definitions there.