Clarify docs for mapState and mapDispatch parameters

Author: markerikson

docs/api.md

@@ -52,13 +52,19 @@ It does not modify the component class passed to it; instead, it *returns* a new
 <a id="connect-arguments"></a>
 #### Arguments
 
-* [`mapStateToProps(state, [ownProps]): stateProps`] \(*Function*): If this argument is specified, the new component will subscribe to Redux store updates. This means that any time the store is updated, `mapStateToProps` will be called. The results of `mapStateToProps` must be a plain object*, which will be merged into the component’s props. If you don't want to subscribe to store updates, pass `null` or `undefined` in place of `mapStateToProps`. If `ownProps` is specified as a second argument, its value will be the props passed to your component, and `mapStateToProps` will be additionally re-invoked whenever the component receives new props (e.g. if props received from a parent component have shallowly changed, and you use the ownProps argument, mapStateToProps is re-evaluated).
+* [`mapStateToProps(state, [ownProps]): stateProps`] \(*Function*): If this argument is specified, the new component will subscribe to Redux store updates. This means that any time the store is updated, `mapStateToProps` will be called. The results of `mapStateToProps` must be a plain object, which will be merged into the component’s props. If you don't want to subscribe to store updates, pass `null` or `undefined` in place of `mapStateToProps`. 
+
+  If your `mapStateToProps` function is declared as taking two parameters, it will be called with the store state as the first parameter and the props passed to the connected component as the second parameter, and will also be re-invoked whenever the connected component receives new props as determined by shallow equality comparisons.  (The second parameter is normally referred to as `ownProps` by convention.)
 
   >Note: in advanced scenarios where you need more control over the rendering performance, `mapStateToProps()` can also return a function. In this case, *that* function will be used as `mapStateToProps()` for a particular component instance. This allows you to do per-instance memoization. You can refer to [#279](https://github.com/reactjs/react-redux/pull/279) and the tests it adds for more details. Most apps never need this.
 
   >The `mapStateToProps` function takes a single argument of the entire Redux store’s state and returns an object to be passed as props. It is often called a **selector**. Use [reselect](https://github.com/reactjs/reselect) to efficiently compose selectors and [compute derived data](http://redux.js.org/docs/recipes/ComputingDerivedData.html).
 
-* [`mapDispatchToProps(dispatch, [ownProps]): dispatchProps`] \(*Object* or *Function*): If an object is passed, each function inside it is assumed to be a Redux action creator. An object with the same function names, but with every action creator wrapped into a `dispatch` call so they may be invoked directly, will be merged into the component’s props. If a function is passed, it will be given `dispatch`. It’s up to you to return an object that somehow uses `dispatch` to bind action creators in your own way. (Tip: you may use the [`bindActionCreators()`](http://reactjs.github.io/redux/docs/api/bindActionCreators.html) helper from Redux.) If you omit it, the default implementation just injects `dispatch` into your component’s props. If `ownProps` is specified as a second argument, its value will be the props passed to your component, and `mapDispatchToProps` will be re-invoked whenever the component receives new props.
+* [`mapDispatchToProps(dispatch, [ownProps]): dispatchProps`] \(*Object* or *Function*): If an object is passed, each function inside it is assumed to be a Redux action creator. An object with the same function names, but with every action creator wrapped into a `dispatch` call so they may be invoked directly, will be merged into the component’s props.  
+
+  If a function is passed, it will be given `dispatch` as the first parameter.  It’s up to you to return an object that somehow uses `dispatch` to bind action creators in your own way. (Tip: you may use the [`bindActionCreators()`](http://reactjs.github.io/redux/docs/api/bindActionCreators.html) helper from Redux.)  If your `mapDispatchToProps` function is declared as taking two parameters, it will be called with `dispatch` as the first parameter and the props passed to the connected component as the second parameter, and will be re-invoked whenever the connected component receives new props.  (The second parameter is normally referred to as `ownProps` by convention.)
+
+  If you do not supply your own `mapDispatchToProps` function or object full of action creators, the default `mapDispatchToProps` implementation just injects `dispatch` into your component’s props.
 
   >Note: in advanced scenarios where you need more control over the rendering performance, `mapDispatchToProps()` can also return a function. In this case, *that* function will be used as `mapDispatchToProps()` for a particular component instance. This allows you to do per-instance memoization. You can refer to [#279](https://github.com/reactjs/react-redux/pull/279) and the tests it adds for more details. Most apps never need this.