Document why constructor is deprecated (#831)

See conversation in #822 (comment) Fixes #822

Author: jonahgraham

org.eclipse.lsp4j/src/main/java/org/eclipse/lsp4j/Protocol.xtend

@@ -104,6 +104,11 @@ class WorkspaceEditCapabilities {
 new() {
 }
 
+/**
+ * @deprecated because when this class first existed it didn't have any other fields.
+ * Now that this class has lots of fields, there is nothing special about this
+ * field and therefore using the setters is the recommended use case.
+ */
 @Deprecated
 new(Boolean documentChanges) {
 this.documentChanges = documentChanges