clarify UnmetDependencyError turning into a CompositeError.

docs/source/parallel/parallel_multiengine.txt

@@ -677,10 +677,8 b' You can also specify imports via the ``@require`` decorator. This is a decorato'
 designed for use in Dependencies, but can be used to handle remote imports as well.
 Modules or module names passed to ``@require`` will be imported before the decorated
 function is called.  If they cannot be imported, the decorated function will never
-execute and will fail with an UnmetDependencyError.
-
-..
-    what about single engines failing to import? will none of them execute the function?
+execute and will fail with an UnmetDependencyError. Failures of single Engines will
+be collected and raise a CompositeError, as demonstrated in the next section.
 
 .. sourcecode:: ipython