Add a comment describing IsolationConfig

client/driver/structs/structs_default.go

@@ -2,5 +2,11 @@
 
 package structs
 
+// IsolationConfig has information about the isolation mechanism the executor
+// uses to put resource constraints and isolation on the user process.  The
+// default implementation is empty.  Platforms that support resource isolation
+// (e.g. Linux's Cgroups) should build their own platform-specific copy.  This
+// information is transmitted via RPC so it is not permissable to change the
+// API.
 type IsolationConfig struct {
 }