Clarify ET can be run standalone

Add note about intended usage.

Author: Kaushik Ghose

Workflow.yml

@@ -177,7 +177,12 @@ $graph:
       specializeTo: ExpressionToolOutputParameter
   documentRoot: true
   doc: |
-    Execute an expression as a Workflow step.
+    An ExpressionTool is a type of Process object that can be run by itself
+    or as part of a Workflow step. It executes a pure Javascript expression
+    that has access to the same input parameters as a workflow. It is meant
+    to be used sparingly as a way to isolate complex javascript expressions
+    that need to operate on input data and produce some result. No docker
+    container is required or allowed.
   fields:
     - name: class
       jsonldPredicate: