DOCSP-17218 what's new in 4.3 (#121)

* Added what's new in 4.3

Author: kyuan-mongodb

source/whats-new.txt

@@ -12,15 +12,64 @@ What's New
 
 Learn what's new in:
 
+* :ref:`Version 4.3 <version-4.3>`
 * :ref:`Version 4.2 <version-4.2>`
 * :ref:`Version 4.1 <version-4.1>`
 * :ref:`Version 4.0 <version-4.0>`
 
+.. _version-4.3:
+
+What's New in 4.3
+-----------------
+
+New features of the 4.3 Java driver release include:
+
+- MongoDB Versioned API. For more information, see our :doc:`Versioned API guide </fundamentals/versioned-api>`. 
+- Added a ``setWindowFields`` pipeline stage to allow the use of window operators 
+- Added support for setting Netty `io.netty.handler.ssl.SslContext <https://netty.io/4.1/api/io/netty/handler/ssl/SslContext.html>`__
+- Added ``snapshot`` (non-speculative) as a ``readConcern`` level for read commands outside of transactions
+- Limited the rate of establishing new connections per connection pool
+- Removed most restrictions on allowed characters in the field names of documents you insert or replace. This is a behavioral change for any application that is relying on client-side enforcement of these restrictions.
+
+The following table shows the restriction status on allowed characters
+in the field names of documents:
+
+.. list-table::
+   :header-rows: 1
+   :stub-columns: 1
+   :widths: 15 25 60
+
+   * - Character
+     - Operation
+     - Description
+
+   * - **.**
+     - Insert and Replace
+     - Removed restrictions on field names containing this character.
+
+   * - **$**
+     - Insert
+     - Removed restrictions on field names starting with this character.
+
+   * - **$**
+     - Replace
+     - Removed restrictions in nested documents on field names containing this character.
+     
+   * - **$**
+     - Replace
+     - Kept restrictions in top-level documents on field names starting with this character. This prevents accidental use of a replace operation when the intention was to use an update operation.
+     
+.. note::
+
+   Unacknowledged writes using dollar-prefixed or dotted keys may
+   be silently rejected by pre-5.0 servers, where some restrictions on
+   field names are still enforced in the serve.
 
 .. _version-4.2:
 
 What's New in 4.2
 -----------------
+
 New features of the 4.2 Java driver release include:
 
 - Added Azure and GCP key stores to client-side field level encryption
@@ -40,6 +89,7 @@ New features of the 4.2 Java driver release include:
 
 What's New in 4.1
 -----------------
+
 New features of the 4.1 Java driver release include:
 
 - Significant reduction in client-perceived failover times during planned maintenance events
@@ -52,4 +102,5 @@ New features of the 4.1 Java driver release include:
 
 What's New in 4.0
 -----------------
+
 This release adds no new features.