Refine docs.

doc/src/safety/sandbox.md

@@ -5,13 +5,31 @@ Sand-Boxing - Block Access to External Data
 
 Rhai is _sand-boxed_ so a script can never read from outside its own environment.
 
-Furthermore, an [`Engine`] created non-`mut` cannot mutate any state outside of itself;
-so it is highly recommended that [`Engine`]'s are created immutable as much as possible.
+Furthermore, an [`Engine`] created non-`mut` cannot mutate any state, including itself
+(and therefore it is also _re-entrant_).
+
+It is highly recommended that [`Engine`]'s be created immutable as much as possible.
 
 ```rust
-let mut engine = Engine::new();     // create mutable 'Engine'
+// Use the fluent API to configure an 'Engine' and then keep an immutable instance.
+let engine = Engine::new()
+               .register_get("field", get_field)
+               .register_set("field", set_field)
+               .register_fn("do_work", action);
 
-engine.register_get("add", add);    // configure 'engine'
-
-let engine = engine;                // shadow the variable so that 'engine' is now immutable
+// 'engine' is immutable...
 ```
+
+
+Using Rhai to Control External Environment
+-----------------------------------------
+
+How does a _sand-boxed_, immutable [`Engine`] control the external environment?
+This is necessary in order to use Rhai as a _dynamic control layer_ over a Rust core system.
+
+There are two general patterns, both involving wrapping the external system
+in a shared, interior-mutated object (e.g. `Rc<RefCell<T>>`):
+
+* [Control Layer]({{rootUrl}}/patterns/control.md) pattern.
+
+* [Singleton Command Object]({{rootUrl}}/patterns/singleton.md) pattern.