docs(slider): clean up docs (#3516)

* clean up slider docs

* add descriptions to MdSliderChange properties

Author: mmalerba

src/lib/slider/slider.html

@@ -1,12 +1,12 @@
 <div class="mat-slider-wrapper">
   <div class="mat-slider-track-wrapper">
-    <div class="mat-slider-track-background" [ngStyle]="trackBackgroundStyles"></div>
-    <div class="mat-slider-track-fill" [ngStyle]="trackFillStyles"></div>
+    <div class="mat-slider-track-background" [ngStyle]="_trackBackgroundStyles"></div>
+    <div class="mat-slider-track-fill" [ngStyle]="_trackFillStyles"></div>
   </div>
-  <div class="mat-slider-ticks-container" [ngStyle]="ticksContainerStyles">
-    <div class="mat-slider-ticks" [ngStyle]="ticksStyles"></div>
+  <div class="mat-slider-ticks-container" [ngStyle]="_ticksContainerStyles">
+    <div class="mat-slider-ticks" [ngStyle]="_ticksStyles"></div>
   </div>
-  <div class="mat-slider-thumb-container" [ngStyle]="thumbContainerStyles">
+  <div class="mat-slider-thumb-container" [ngStyle]="_thumbContainerStyles">
     <div class="mat-slider-thumb"></div>
     <div class="mat-slider-thumb-label">
       <span class="mat-slider-thumb-label-text">{{displayValue}}</span>