[T] Document downstream IG rules for element prohibitions conflicting with inherited Must Support/Obligations

[niccoreyes]

Task Description

Clarify and document the rules for downstream Implementation Guides that inherit PH Core profiles and need to prohibit certain elements (cardinality 0..0) that carry inherited Must Support flags and Obligation extensions.

Specifically, document how the FHIR Obligation ignore modifier should be used when a downstream IG wants to carve out an inherited MS+Obligation element from its server support.

Rationale

This was surfaced during downstream IG development where they proposed 0..0 on the following elements inherited from PH Core:

• implicitRules, language, text, contained, modifierExtension
• multipleBirth[x], communication, managingOrganization, link
• Several sub-elements of name, telecom, and contact

Under current PH Core heuristics (established by PR #213 — Obligations migration), these elements carry Must Support + Obligation extensions (e.g., ObligationOptional). Setting them to 0..0 in a downstream derivative creates a conformance conflict: the element is both MS (inherited) and prohibited (derived).

The FHIR Obligations extension provides an ignore code for this purpose (see Obligation CodeSystem), but PH Core currently has no documented policy for:

1. When and how downstream IGs should use the ignore obligation modifier
2. Whether 0..0 cardinality alone is sufficient to override inherited MS
3. Whether downstream IGs need explicit PH Core team approval before applying ignore
4. How this affects IG publisher validation and QA reports

Expected Outcome

• Documented policy in CONTRIBUTING.md or a dedicated IG page covering downstream override rules
• Obligation ignore usage pattern with FSH example (e.g., * element insert ObligationIgnore RuleSet)
• Guidance on when 0..0 cardinality is sufficient vs. when explicit ignore obligation is required
• Clear statement that upstream PH Core must approve any ignore carve-outs to maintain national consistency
• Decision logged in the Decision Log section of CONTRIBUTING.md

Additional Context

• PH Core Obligations: PR [FR-U] Refactor: Migrate from Must Support (MS) flags to FHIR Obligations extension #213 — migrated PH Core from MS-only to Obligations extension model
• Related: Issue [T] Add CI/CD automation to catch Orphan MS items #301 — CI/CD automation to catch Orphan MS items (no obligation declared)
• Obligation ignore reference: https://hl7.org/fhir/extensions/5.3.0/CodeSystem-obligation.html#obligation-ignore
• Heuristic to review: PH Core is a foundation, not a ceiling (CONTRIBUTING.md § Key Concepts) — downstream IGs can constrain further, but the mechanism for doing so must be explicit
• Actors to consider: Downstream IG authors, PH Core governance (TWG), IG publisher tooling