docs: clarify http response type (#32817)

PR Close #32817

packages/common/http/src/client.ts

@@ -20,6 +20,12 @@ import {HttpEvent, HttpResponse} from './response';
 /**
  * Constructs an instance of `HttpRequestOptions<T>` from a source `HttpMethodOptions` and
  * the given `body`. This function clones the object and adds the body.
+ *
+ * Note that the `responseType` *options* value is a String that identifies the
+ * single data type of the response.
+ * A single overload version of the method handles each response type.
+ * The value of `responseType` cannot be a union, as the combined signature could imply.
+ *
  */
 function addBody<T>(
     options: {
@@ -46,10 +52,15 @@ export type HttpObserve = 'body' | 'events' | 'response';
 
 /**
  * Performs HTTP requests.
- *
  * This service is available as an injectable class, with methods to perform HTTP requests.
  * Each request method has multiple signatures, and the return type varies based on
  * the signature that is called (mainly the values of `observe` and `responseType`).
+ *
+ * Note that the `responseType` *options* value is a String that identifies the
+ * single data type of the response.
+ * A single overload version of the method handles each response type.
+ * The value of `responseType` cannot be a union, as the combined signature could imply.
+
  *
  * @usageNotes
  * Sample HTTP requests for the [Tour of Heroes](/tutorial/toh-pt0) application.