Extract copy/restore of first argument in method call.

README.md

@@ -47,7 +47,7 @@ It doesn't attempt to be a new language. For example:
 
 * No classes.  Well, Rust doesn't either. On the other hand...
 * No traits...  so it is also not Rust. Do your Rusty stuff in Rust.
-* No structures - definte your types in Rust instead; Rhai can seamless work with _any Rust type_.
+* No structures - define your types in Rust instead; Rhai can seamlessly work with _any Rust type_.
 * No first-class functions - Code your functions in Rust instead, and register them with Rhai.
 * No closures - do your closure magic in Rust instead; [turn a Rhai scripted function into a Rust closure](#calling-rhai-functions-from-rust).
 * It is best to expose an API in Rhai for scripts to call.  All your core functionalities should be in Rust.
@@ -2094,14 +2094,24 @@ Members and methods
 -------------------
 
 Properties and methods in a Rust custom type registered with the [`Engine`] can be called just like in Rust.
+Unlike functions defined in script (for which all arguments are passed by _value_),
+native Rust functions may mutate the object (or the first argument if called in normal function call style).
 
 ```rust
 let a = new_ts();           // constructor function
-a.field = 500;              // property access
-a.update();                 // method call, 'a' can be changed
+a.field = 500;              // property setter
+a.update();                 // method call, 'a' can be modified
 
-update(a);                  // this works, but 'a' is unchanged because only
-                            // a COPY of 'a' is passed to 'update' by VALUE
+update(a);                  // <- this de-sugars to 'a.update()' this if 'a' is a simple variable
+                            //    unlike scripted functions, 'a' can be modified and is not a copy
+
+let array = [ a ];
+
+update(array[0]);           // <- 'array[0]' is an expression returning a calculated value,
+                            //    a transient (i.e. a copy) so this statement has no effect
+                            //    except waste a lot of time cloning
+
+array[0].update();          // <- call this method-call style will update 'a'
 ```
 
 Custom types, properties and methods can be disabled via the [`no_object`] feature.