doc: more use-cases for promise events

This adds an example of a semi-common promise pattern that could
result in false-positive 'unhandledRejection' events, to help motivate
why there is a dual 'rejectionHandled' event. I anticipate it being
helpful to users who encounter 'unhandledRejection' events that do not
stem from obvious typos such as the JSON.pasre example.

Also cleans up the promise rejection tracking sample. By using a Set
instead of array we can get O(1) deletion. And, fixed indentation there
to align with the rest of the document.

doc/api/process.markdown

@@ -137,13 +137,29 @@ Here is an example that logs every unhandled rejection to the console
         // application specific logging, throwing an error, or other logic here
     });
 
-For example, here is a rejection that will trigger the `'unhandledRejection'`
+For example, here is a rejection that will trigger the 'unhandledRejection'
 event:
 
     somePromise.then(function(res) {
       return reportToUser(JSON.pasre(res)); // note the typo
     }); // no `.catch` or `.then`
 
+Here is an example of a coding pattern that will also trigger
+'unhandledRejection':
+
+    function SomeResource() {
+      // Initially set the loaded status to a rejected promise
+      this.loaded = Promise.reject(new Error('Resource not yet loaded!'));
+    }
+
+    var resource = new SomeResource();
+    // no .catch or .then on resource.loaded for at least a turn
+
+To deal with cases like this, which are likely false positives, you can either
+attach a dummy `.catch(function() { })` handler to `resource.loaded`,
+preventing the 'unhandledRejection' event from being emitted, or you can use
+the 'rejectionHandled' event.
+
 ## Event: 'rejectionHandled'
 
 Emitted whenever a Promise was rejected and an error handler was attached to it
@@ -169,15 +185,18 @@ event tells you when the list of unhandled rejections shrinks.
 For example using the rejection detection hooks in order to keep a list of all
 the rejected promises at a given time:
 
-    var unhandledRejections = [];
+    var unhandledRejections = new Set();
     process.on('unhandledRejection', function(reason, p) {
-        unhandledRejections.push(p);
+      unhandledRejections.add(p);
     });
     process.on('rejectionHandled', function(p) {
-        var index = unhandledRejections.indexOf(p);
-        unhandledRejections.splice(index, 1);
+      unhandledRejections.delete(p);
     });
 
+You could then record this list in some error log, either periodically or upon
+process exit. This would avoid the false positive seen in the above resource
+example.
+
 ## Signal Events
 
 <!--type=event-->