Update changelog for 9.0.0 body param removal (#2523)

Author: JoshMock

docs/changelog.asciidoc

@@ -1,6 +1,17 @@
 [[changelog-client]]
 == Release notes
 
+[discrete]
+=== 9.0.0
+
+[discrete]
+==== Breaking changes
+
+[discrete]
+===== Drop support for deprecated `body` parameter
+
+In 8.0, the top-level `body` parameter that was available on all API functions <<remove-body-key,was deprecated>>. In 9.0 this property is completely removed.
+
 [discrete]
 === 8.16.2
 
@@ -645,6 +656,7 @@ ac.abort()
 ----
 
 [discrete]
+[[remove-body-key]]
 ===== Remove the body key from the request
 
 *Breaking: Yes* | *Migration effort: Small*