Document the profiles field on VolumeSpec. (#2709)

docs/pages/yaml-reference.md

@@ -442,6 +442,10 @@ This documentation effectively reflects the Java object tree under [RawServiceSp
         <li><code>MOUNT</code> volumes are separate partitions which the cluster administrator had mounted onto the host machine as <code>/dcos/volumeN</code>. These partitions will typically have their own dedicated IO/spindles, resulting in more consistent performance. <code>MOUNT</code> volumes are reserved as a unit and are not shared across services. If a service requests a 1 GB volume and the <code>MOUNT</code> volumes are all 100 GB, then the service is getting a 100 GB volume all to itself.</li>
         </ul></div>
 
+      * `profiles`
+
+        A list of [volume profiles](https://docs.mesosphere.com/services/beta-storage/0.4.0-beta/terminology-and-concepts/#volume-profile) any of which the persistent volume should be provisioned on. This is only supported for `MOUNT` volumes in DC/OS 1.12+.
+
       * `size`
 
         The required minimum size of the volume. See reservation semantics between `ROOT` and `MOUNT` volume types above.