Add links to docs for addons and top level API (#6555)

This makes it easier to figure out where the docs live.

Googling for e.g. `react-addons-update` also works, but this should
make things easier for people that hyperclick directly to the source.

src/test/ReactPerf.js

@@ -14,6 +14,7 @@
 /**
  * ReactPerf is a general AOP system designed to measure performance. This
  * module only has the hooks: see ReactDefaultPerf for the analysis tool.
+ * See also https://facebook.github.io/react/docs/perf.html
  */
 var ReactPerf = {
   /**

src/test/ReactTestUtils.js

@@ -71,6 +71,10 @@ function findAllInRenderedTreeInternal(inst, test) {
 }
 
 /**
+ * Utilities for making it easy to test React components.
+ *
+ * See https://facebook.github.io/react/docs/test-utils.html
+ *
  * Todo: Support the entire DOM.scry query syntax. For now, these simple
  * utilities will suffice for testing purposes.
  * @lends ReactTestUtils