Switching to a more popular representation of optional arguments

content/docs/reference-react-component.md

@@ -291,7 +291,7 @@ For more details, see [*Error Handling in React 16*](/blog/2017/07/26/error-hand
 ### `setState()`
 
 ```javascript
-setState(updater, [callback])
+setState(updater[, callback])
 ```
 
 `setState()` enqueues changes to the component state and tells React that this component and its children need to be re-rendered with the updated state. This is the primary method you use to update the user interface in response to event handlers and server responses.
@@ -323,7 +323,7 @@ The second parameter to `setState()` is an optional callback function that will
 You may optionally pass an object as the first argument to `setState()` instead of a function:
 
 ```javascript
-setState(stateChange, [callback])
+setState(stateChange[, callback])
 ```
 
 This performs a shallow merge of `stateChange` into the new state, e.g., to adjust a shopping cart item quantity:

content/docs/reference-react-dom.md

@@ -33,11 +33,7 @@ React supports all popular browsers, including Internet Explorer 9 and above.
 ### `render()`
 
 ```javascript
-ReactDOM.render(
-  element,
-  container,
-  [callback]
-)
+ReactDOM.render(element, container[, callback])
 ```
 
 Render a React element into the DOM in the supplied `container` and return a [reference](/docs/more-about-refs.html) to the component (or returns `null` for [stateless components](/docs/components-and-props.html#functional-and-class-components)).
@@ -63,11 +59,7 @@ If the optional callback is provided, it will be executed after the component is
 ### `hydrate()`
 
 ```javascript
-ReactDOM.hydrate(
-  element,
-  container,
-  [callback]
-)
+ReactDOM.hydrate(element, container[, callback])
 ```
 
 Same as [`render()`](#render), but is used to hydrate a container whose HTML contents were rendered by [`ReactDOMServer`](/docs/react-dom-server.html). React will attempt to attach event listeners to the existing markup.