Add Rhai book.

doc/src/language/method.md

@@ -0,0 +1,28 @@
+Call Method as Function
+======================
+
+{{#include ../links.md}}
+
+Properties and methods in a Rust custom type registered with the [`Engine`] can be called just like a regular function 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).
+
+Custom types, properties and methods can be disabled via the [`no_object`] feature.
+
+```rust
+let a = new_ts();   // constructor function
+a.field = 500;      // property setter
+a.update();         // method call, 'a' can be modified
+
+update(a);          // <- this de-sugars to 'a.update()' thus 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'
+```