(SCHEMA) Update returnKind and associated properties

Prior to this change, the `returnKind` shared definition set a default
value. This was inaccurate, because `set.return` has different behavior
when `return` isn't defined - DSC invokes the **Get** operation to
construct the result after the **Set** operation for the resource
concludes.

This change removes the default from the shared definition, adds it
to the `test.return` property, and clarifies the behavior for the
`set.return` property. Finally, this change also makes `whatIf.return`
mandatory, as the synthetic after-state doesn't make sense for this
operation, even if DSC will technically accept it.

Author: michaeltlombardi

schemas/src/definitions/returnKind.yaml

@@ -7,7 +7,6 @@ type: string
 enum:
   - state
   - stateAndDiff
-default: state
 
 $comment: |-
   While the enumeration for return kind is the same for the `set` and `test`

schemas/src/resource/manifest.set.yaml

@@ -179,6 +179,10 @@ properties:
       Defines whether the command returns a JSON blob of the DSC resource's state after the set
       operation or the state and an array of the properties the DSC resource modified.
 
+      When a manifest doesn't define `set.return`, DSC doesn't expect the resource to emit any
+      JSON to stdout. Instead, DSC invokes the **Get** operation for the resource to construct
+      the result object, retrieving the actual state of the resource after the **Set** operation.
+
       [01]: <DOCS_BASE_URL>/reference/schemas/resource/manifest/set?<DOCS_VERSION_PIN>#return
     markdownEnumDescriptions:
       - | # state

schemas/src/resource/manifest.test.yaml

@@ -133,6 +133,7 @@ properties:
       Defines whether the command returns a JSON blob of the DSC resource's current state or the
       state and an array of the properties that are out of the desired state.
     $ref: /<PREFIX>/<VERSION>/definitions/returnKind.yaml
+    default: state
     # VS Code only
     markdownDescription: |
       ***

schemas/src/resource/manifest.whatIf.yaml

@@ -40,6 +40,7 @@ markdownDescription: | # VS Code only
 type: object
 required:
   - executable
+  - return
 properties:
   executable:
     $ref: /<PREFIX>/<VERSION>/definitions/commandExecutable.yaml
@@ -190,6 +191,15 @@ properties:
       set operation in what-if mode or the state and an array of the properties the DSC resource
       would modify.
 
+      Unlike the `set.return` property, you must specify a value for `whatIf.return` for the
+      resource to function correctly. When you don't define `set.return` in your resource manifest,
+      DSC invokes the **Get** operation on the resource after the **Set** operation to construct
+      the result with the final state of the resource.
+
+      For what-if operations, the resource doesn't actually change the system, so invoking the
+      **Get** operation after the what-if command executes will always show the final state to
+      be the same as the before state.
+
       [01]: <DOCS_BASE_URL>/reference/schemas/resource/manifest/whatif?<DOCS_VERSION_PIN>#return
     markdownEnumDescriptions:
       - | # state