When a server validates a complex code - e.g. a grammar - it might not perform all the validation steps that are possible. E.g. tx.fhir.org does not (yet) validate snomed expressions against the MRCM. We should document with an example how to inform the user of what validation limitations they should be aware of, using an issue with a tx-issue-type of ```processing-note``` (see https://chat.fhir.org/#narrow/stream/179166-implementers/topic/The.20URL.20is.20not.20valid.20-.20Snomed-ct.20compositional.20grammar.20.7B.20.7D/near/421727912)