86016: Add typedocs

src/app/correlation-id/correlation-id.reducer.ts

@@ -9,6 +9,12 @@ const initialState = null;
 
 export const correlationIdSelector = (state: AppState) => state.correlationId;
 
+/**
+ * Reducer that handles actions to update the correlation ID
+ * @param {string} state                the previous correlation ID (null if unset)
+ * @param {CorrelationIdAction} action  the action to perform
+ * @return {string}                     the new correlation ID
+ */
 export const correlationIdReducer = (state = initialState, action: CorrelationIdAction): string => {
   switch (action.type) {
     case CorrelationIDActionTypes.SET: {