Rewrite docs for getters/setters

Author: alexcrichton

guide/src/reference/attributes/on-rust-exports/getter-and-setter.md

@@ -1,33 +1,27 @@
 # `getter` and `setter`
 
-It is also possible to interact with `Rust` types either by using fields accessors. For example, the following:
+The `getter` and `setter` attributes can be used in Rust `impl` blocks to define
+properties in JS that act like getters and setters of a field. For example:
 
 ```rust
 #[wasm_bindgen]
-extern "C" {
-    fn check_modify_and_return_baz_in_js_fields(baz: Baz) -> Baz;
-}
-
-#[wasm_bindgen_test]
-fn create_and_check_baz_in_rust() {
-    let baz = check_modify_and_return_baz_in_js_fields(Baz { field: 123 });
-    assert_eq!(baz.field.unwrap(), 456);
-}
-
-#[wasm_bindgen]
-#[derive(Default)]
 pub struct Baz {
     field: i32,
 }
 
 #[wasm_bindgen]
 impl Baz {
+    #[wasm_bindgen(constructor)]
+    pub fn new(field: i32) -> Baz {
+        Baz { field }
+    }
+
     #[wasm_bindgen(getter)]
     pub fn field(&self) -> i32 {
         self.field
     }
 
-    #[wasm_bindgen(setter = field)]
+    #[wasm_bindgen(setter)]
     pub fn set_field(&mut self, field: i32) {
         self.field = field;
     }
@@ -37,9 +31,34 @@ impl Baz {
 Can be combined in `JavaScript` like in this snippet:
 
 ```js
-check_modify_and_return_baz_in_js_fields = (baz) => {
-    console.log(baz.field, 123);
-    baz.field = 456;
-    return baz;
-};
-```
+const obj = new Baz(3);
+assert.equal(obj.field, 3);
+obj.field = 4;
+assert.equal(obj.field, 4);
+```
+
+You can also configure the name of the property that is exported in JS like so:
+
+```rust
+#[wasm_bindgen]
+impl Baz {
+    #[wasm_bindgen(getter = anotherName)]
+    pub fn field(&self) -> i32 {
+        self.field
+    }
+
+    #[wasm_bindgen(setter = anotherName)]
+    pub fn set_field(&mut self, field: i32) {
+        self.field = field;
+    }
+}
+```
+
+Getters are expected to take no arguments other than `&self` and return the
+field's type. Setters are expected to take one argument other than `&mut self`
+(or `&self`) and return no values.
+
+The name for a `getter` is by default inferred from the function name it's
+attached to. The default name for a `setter` is the function's name minus the
+`set_` prefix, and if `set_` isn't a prefix of the function it's an error to not
+provide the name explicitly.