[formrecognizer] Update changelog for release (#20963)

* update changelog for release

* include snippet about new clients

sdk/formrecognizer/azure-ai-formrecognizer/CHANGELOG.md

@@ -1,9 +1,11 @@
 # Release History
 
-## 3.2.0b1 (Unreleased)
+## 3.2.0b1 (2021-10-05)
 
 This version of the SDK defaults to the latest supported API version, which currently is v2021-09-30-preview.
 
+> Note: Starting with version 2021-09-30-preview, a new set of clients were introduced to leverage the newest features of the Form Recognizer service. Please see the Migration Guide for detailed instructions on how to update application code from client library version 3.1.X or lower to the latest version. Also, please refer to the README for more information about the library. 
+
 ### Features Added
 - Added new `DocumentAnalysisClient` with  `begin_analyze_document` and `begin_analyze_document_from_url` methods. Use these methods with the latest Form Recognizer 
 API version to analyze documents, with prebuilt and custom models.
@@ -13,10 +15,6 @@ API version to analyze documents, with prebuilt and custom models.
 - Added samples using the `DocumentAnalysisClient` and `DocumentModelAdministrationClient` under `/samples/v3.2-beta`.
 - Added `DocumentAnalysisApiVersion` to be used with `DocumentAnalysisClient` and `DocumentModelAdministrationClient`.
 
-### Breaking Changes
-
-### Bugs Fixed
-
 ### Other Changes
 - Python 3.5 is no longer supported in this release.