[JAVA] Improve map/flatmap of Publisher[Void] (#499)

* what's new: flatmap and observables

Author: rachel-mack

source/upgrade.txt

@@ -104,6 +104,18 @@ Version 5.0 Breaking Changes
 
      VectorSearchOptions opts = vectorSearchOptions().filter(eq("x", 8));
 
+.. _java-breaking-changes-v5.0-observables:
+
+- This driver version removes the
+  ``org.mongodb.scala.ObservableImplicits.ToSingleObservableVoid`` implicit
+  class. This means the ``org.reactivestreams.Publisher[Void]`` type no longer
+  converts automatically to ``org.mongodb.scala.SingleObservable[Void]``. The also
+  API exposes ``org.mongodb.scala.Observable[Unit]`` instead of
+  ``org.mongodb.scala.Observable[Void]``. 
+  
+  .. After the 5.0 Scala API docs are released, this line will be uncommented. 
+    For more information, see the `Observable trait documentation <https://mongodb.github.io/mongo-java-driver/5.0/apidocs/mongo-scala-driver/org/mongodb/scala/Observable.html>`__.
+
 .. _java-breaking-changes-v4.8:
 
 Version 4.8 Breaking Changes

source/whats-new.txt

@@ -59,6 +59,10 @@ The 5.0 driver release introduces the following features:
   ``isNumber()`` and ``asNumber()`` methods match the Java responses for
   equivalent ``Decimal128`` values.
 
+- The ``org.mongodb.scala.Observable.completeWithUnit()`` method is deprecated.
+  This relates to a :ref:`breaking change about Observables in this version
+  <java-breaking-changes-v5.0-observables>`.
+
 .. _version-4.11:
 
 What's New in 4.11