Clarify this section about what  server and client expectation are around non-must supported mandatory elements.

Additional guidance like:"Receivers must check to see if unexpected modifiers are present and must either reject or figure out if they can safely process.  (A modifier nested inside something you don't care about can potentially be ignored.  If the instance is for human-review only, you might be able to ignore.  Otherwise, rejection is usually the only safe action.)"

or similar guidance published in US Core 6.1.0 : https://hl7.org/fhir/us/core/general-requirements.html#modifier-elements