HDATA — Validation service

Similar to the version-aware resource structure, the following URL for
sectionDocuments is reserved:

(baseURL)/(sectionPath)/(sectionDocument)/validate/

This allows a client to ask the server whether the attached POSTed
content would be
acceptable as a PUT. By this means that client can
(a) validate business logic during testing concerning correct content
(b) implement a lite two-phase commit alternative that reduces the
chance of partial success of multi-resource operations.

The second functionality overlaps a little with existing functionality
around safe commit,
though what is there now solves a different problem. From the point of
view of FHIR, the
first is at least as important: a real world way to test content
without only having to rely
on claims.

Added text defining this mechanism to section 6.5.