Reject null for non-optional arguments

Closes gh-33339

framework-docs/modules/ROOT/pages/integration/rest-clients.adoc

@@ -978,6 +978,9 @@ method parameters:
   `MultiValueMap<String, ?>` with multiple cookies, a `Collection<?>` of values, or an
   individual value. Type conversion is supported for non-String values.
 
+The parameters can't be null unless they are set as not required through the annotation,
+annotated with `@Nullable` or they are `Optional`.
+
 |===
 
 

framework-docs/modules/ROOT/pages/rsocket.adoc

@@ -1115,7 +1115,8 @@ method parameters:
 | `@Payload`
 | Set the input payload(s) for the request. This can be a concrete value, or any producer
   of values that can be adapted to a Reactive Streams `Publisher` via
-  `ReactiveAdapterRegistry`
+  `ReactiveAdapterRegistry`. The payload can't be null unless it's set as not required
+through the annotation, annotated with `@Nullable` or it is `Optional`.
 
 | `Object`, if followed by `MimeType`
 | The value for a metadata entry in the input payload. This can be any `Object` as long