Merge pull request #4469 from KevinTCoughlin/kevintcoughlin/#4265

Fixes #4265 - Clarify React.render note in documentation

docs/ref-01-top-level-api.md

@@ -89,9 +89,13 @@ If the optional callback is provided, it will be executed after the component is
 
 > Note:
 >
-> `React.render()` replaces the contents of the container node you
-> pass in. In the future, it may be possible to insert a component to an
-> existing DOM node without overwriting the existing children.
+> `React.render()` controls the contents of the container node you pass in. Any existing DOM elements
+> inside are replaced when first called. Later calls use React’s DOM diffing algorithm for efficient
+> updates.
+>
+> `React.render()` does not modify the container node (only modifies the children of the container). In
+> the future, it may be possible to insert a component to an existing DOM node without overwriting
+> the existing children.
 
 
 ### React.unmountComponentAtNode