Clarify docs on target-framerate. Fixes #4897

lib/elements/dom-repeat.html

@@ -250,12 +250,16 @@
         },
 
         /**
-         * When `initialCount` is used, this property defines a frame rate to
-         * target by throttling the number of instances rendered each frame to
-         * not exceed the budget for the target frame rate.  Setting this to a
+         * When `initialCount` is used, this property defines a frame rate (in
+         * fps) to target by throttling the number of instances rendered each
+         * frame to not exceed the budget for the target frame rate.  The
+         * framerate is effectively the number of `requestAnimationFrame`s that
+         * it tries to allow to actually fire in a given second, and it targets
+         * this by empirically measuring the time between rAF's and throttling
+         * up/down the number of items created each `rAF`. Setting this to a 
          * higher number will allow lower latency and higher throughput for
-         * things like event handlers, but will result in a longer time for the
-         * remaining items to complete rendering.
+         * event handlers and other tasks, but will result in a longer time for
+         * the remaining items to complete rendering.
          */
         targetFramerate: {
           type: Number,