DFDL extends a subset of XML Schema. Note however: an XML Schema is an XML document.
There are XML-Schema-Aware editors. So, if you have an editor that is aware of the XML Schema of an XML document, then that editor can provide intelligent assistance in authoring the XML document. It can show you the XML source code, or can show you diagrammatic ways of examining the document.
Now, turns out there is an XML Schema for DFDL Schemas. It was created by taking the XML Schema for XML Schemas, and subsetting it to just what DFDL uses, and then adding into it the XML Schema for DFDL's annotation objects.
Since there is an XML Schema for the XML Documents that just happen to be DFDL Schemas, an XML aware editor can provide assistance in authoring DFDL Schemas.
The Eclipse Settings page is instructions for setting up the Eclipse Integrated-Development-Environment (IDE), and to configure it's XML editing features so that they provide some level of support for authoring DFDL schemas. In theory, based on this, if you have some other favorite XML-Schema-Aware editing environment, you could figure out how to enable it to provide you similar DFDL schema authoring assistance.
See Eclipse Settings.