Update docs for CoreSearchEngine.ApiType.toSDKType (tests only)

Tests/MapboxSearchTests/Common/Extensions/CoreApiType+ToSDKType.swift

@@ -1,6 +1,15 @@
 @testable import MapboxSearch
 
 extension CoreSearchEngine.ApiType {
+    /// Available only for tests.
+    /// This is necessary for tests using subclasses and usages of ``AbstractSearchEngine`` to dynamically instantiate
+    /// an engine with the appropriate ApiType for their test case mocks.
+    /// ``AbstractSearchEngine`` and subclasses use a ``MapboxSearch.ApiType`` enum which is a **subset** of the Core
+    /// ApiType.
+    /// For all other ApiTypes specialized classes are provided that use the Core ApiType directly (such as
+    /// AddressAutofill) or support is not yet fully implemented (such as search-box).
+    /// Tests that attempt to use an Autofill or Search-box API type with a custom SearchEngine (instead of a provided
+    /// specialized class) should encounter a runtime error.
     func toSDKType() -> ApiType? {
         switch self {
         case .geocoding:
@@ -9,7 +18,7 @@ extension CoreSearchEngine.ApiType {
             return .SBS
         case .autofill,
              .searchBox:
-            return nil
+            fallthrough
         @unknown default:
             fatalError()
         }