terraform: update comment on interpolater stuff

terraform/eval_context_builtin.go

@@ -12,10 +12,20 @@ import (
 // BuiltinEvalContext is an EvalContext implementation that is used by
 // Terraform by default.
 type BuiltinEvalContext struct {
-	PathValue           []string
+	// PathValue is the Path that this context is operating within.
+	PathValue []string
+
+	// Interpolater setting below affect the interpolation of variables.
+	//
+	// The InterpolaterVars are the exact value for ${var.foo} values.
+	// The map is shared between all contexts and is a mapping of
+	// PATH to KEY to VALUE. Because it is shared by all contexts as well
+	// as the Interpolater itself, it is protected by InterpolaterVarLock
+	// which must be locked during any access to the map.
 	Interpolater        *Interpolater
 	InterpolaterVars    map[string]map[string]string
 	InterpolaterVarLock *sync.Mutex
+
 	Hooks               []Hook
 	InputValue          UIInput
 	Providers           map[string]ResourceProviderFactory