Revise docs.

doc/src/language/method.md

@@ -37,12 +37,30 @@ array[0].update();  // <- call in method-call style will update 'a'
 ```
 
 
-`&mut` is Efficient
-------------------
+Number of Parameters
+--------------------
+
+Native Rust methods registered with an [`Engine`] take _one additional parameter_ more than
+an equivalent method coded in script, where the object is accessed via the `this` pointer instead.
+
+The following table illustrates the differences:
+
+| Function type | Parameters |    Object reference    |                   Function signature                    |
+| :-----------: | :--------: | :--------------------: | :-----------------------------------------------------: |
+|  Native Rust  |  _n_ + 1   | First `&mut` parameter | `fn method<T, U, V>`<br/>`(obj: &mut T, x: U, y: V) {}` |
+|  Rhai script  |    _n_     |         `this`         |                  `fn method(x, y) {}`                   |
+
+
+`&mut` is Efficient (Except for `ImmutableString`)
+------------------------------------------------
 
 Using a `&mut` first parameter is highly encouraged when using types that are expensive to clone,
 even when the intention is not to mutate that argument, because it avoids cloning that argument value.
 
+For example, the `len` method of an [array] has the signature: `Fn(&mut Array) -> INT`.
+The array itself is not modified in any way, but using a `&mut` parameter avoids a cloning that would
+otherwise have happened if the signature were `Fn(Array) -> INT`.
+
 For primary types that are cheap to clone (e.g. those that implement `Copy`),
 including `ImmutableString`, this is not necessary.