Add doc about passing answers from Questionnaire A to B (#3272)

docs/engineering/writing-fhir/form-authoring.mdx

@@ -1,5 +1,6 @@
-# Form Authoring (Things to consider when writing forms)
+# Form Authoring
 
+## Things to consider when writing forms
 - Make sure that the `enableWhen` expressions are accompanied by the `enableBehaviour` attribute in order to avoid form crashes.
 - The `enableBehaviour` defines whether the expressions on the `enableWhen` are to use `and` or `or` logic.
 - The `enableBehaviour` options are `any` for `or` & `all` for `and`.
@@ -34,4 +35,38 @@
   ],
   "enableBehaviour":"all"
 }
-```
+```
+
+## How to pass answers/data from Questionnaire A to B
+1. Submit Questionnaire A
+2. In Questionnaire B, use X-FHIR-Query variable to search the Questionnaire Response of Questionnaire A that also belongs to the Patient that did the submission
+```
+{
+  "url": "http://hl7.org/fhir/StructureDefinition/variable",
+  "valueExpression": {
+    "name": "qr-search-phq9",
+    "language": "application/x-fhir-query",
+    "expression": "QuestionnaireResponse?subject={{%patient.id.replaceMatches('/_history/.*', '')}}&questionnaire=Questionnaire/57"
+  }
+},
+{
+  "url": "http://hl7.org/fhir/StructureDefinition/variable",
+  "valueExpression": {
+    "name": "qr-phq9",
+    "language": "text/fhirpath",
+    "expression": "%qr-search-phq9.entry.first().resource"
+  }
+}
+```
+3. In Questionnaire B, get the answer of Questionnaire A through the variable that contains Questionnaire Response of Questionnaire A
+```
+{
+  "url": "http://hl7.org/fhir/StructureDefinition/variable",
+  "valueExpression": {
+    "name": "phq9-p1",
+    "language": "text/fhirpath",
+    "expression": "%qr-phq9.item.descendants().where(linkId='f1.3.1').answer.value"
+  }
+}
+```
+4. Call that variable in a calculated-expression so it can be populated into the field