doc: document the tracing api

Author: tjfontaine

doc/api/_toc.markdown

@@ -29,6 +29,7 @@
 * [String Decoder](string_decoder.html)
 * [Timers](timers.html)
 * [TLS/SSL](tls.html)
+* [Tracing](tracing.html)
 * [TTY](tty.html)
 * [UDP/Datagram](dgram.html)
 * [URL](url.html)

doc/api/all.markdown

@@ -35,3 +35,4 @@
 @include debugger
 @include cluster
 @include smalloc
+@include tracing

doc/api/tracing.markdown

@@ -0,0 +1,61 @@
+# Tracing
+
+    Stability: 1 - Experimental
+
+The tracing module is designed for instrumenting your Node application. It is
+not meant for general purpose use.
+
+***Be very careful with callbacks used in conjunction with this module***
+
+Many of these callbacks interact directly with asynchronous subsystems in a
+synchronous fashion. That is to say, you may be in a callback where a call to
+`console.log()` could result in an infinite recursive loop.  Also of note, many
+of these callbacks are in hot execution code paths. That is to say your
+callbacks are executed quite often in the normal operation of Node, so be wary
+of doing CPU bound or synchronous workloads in these functions. Consider a ring
+buffer and a timer to defer processing.
+
+`require('tracing')` to use this module.
+
+## v8
+
+The `v8` property is an [EventEmitter][], it exposes events and interfaces
+specific to the version of `v8` built with node. These interfaces are subject
+to change by upstream and are therefore not covered under the stability index.
+
+### Event: 'gc'
+
+`function (before, after) { }`
+
+Emitted each time a GC run is completed.
+
+`before` and `after` are objects with the following properties:
+
+```
+{
+  type: 'mark-sweep-compact',
+  flags: 0,
+  timestamp: 905535650119053,
+  total_heap_size: 6295040,
+  total_heap_size_executable: 4194304,
+  total_physical_size: 6295040,
+  used_heap_size: 2855416,
+  heap_size_limit: 1535115264
+}
+```
+
+### getHeapStatistics()
+
+Returns an object with the following properties
+
+```
+{
+  total_heap_size: 7326976,
+  total_heap_size_executable: 4194304,
+  total_physical_size: 7326976,
+  used_heap_size: 3476208,
+  heap_size_limit: 1535115264
+}
+```
+
+[EventEmitter]: events.html#events_class_events_eventemitter