Add some docs

Author: armanbilge

dom/src/main/scala/org/scalajs/dom/PerformanceEntry.scala

@@ -9,14 +9,26 @@ package org.scalajs.dom
 import scala.scalajs.js
 import scala.scalajs.js.annotation._
 
+/** Encapsulates a single performance metric that is part of the browser's performance timeline. */
 @js.native
 @JSGlobal
 class PerformanceEntry extends js.Object {
+
+  /** The name for a performance entry. It acts as an identifier, but it does not have to be unique. The value depends
+    * on the subclass.
+    */
   def name: String = js.native
 
+  /** The first timestamp recorded for this performance entry. The meaning of this property depends on the value of this
+    * entry's [[entryType]].
+    */
   def startTime: Double = js.native
 
+  /** The duration of the performance entry. The meaning of this property depends on the value of this entry's
+    * [[entryType]].
+    */
   def duration: Int = js.native
 
+  /** The type of performance metric that this entry represents. */
   def entryType: String = js.native
 }