Added missing/corrected malformed remarks sections in Aspect.xml (#954)

* Added missing/corrected malformed remarks sections in Aspect.xml

docs/Xamarin.Forms.Core/Xamarin.Forms/Aspect.xml

@@ -16,8 +16,7 @@
   </Base>
   <Docs>
     <summary>Defines how an image is displayed.</summary>
-    <remarks>
-    </remarks>
+    <remarks>To be added.</remarks>
   </Docs>
   <Members>
     <Member MemberName="AspectFill">
@@ -38,8 +37,7 @@
       </ReturnValue>
       <Docs>
         <summary>Scale the image to fill the view. Some parts may be clipped in order to fill the view.</summary>
-        <remarks>
-        </remarks>
+        <remarks>To be added.</remarks>
       </Docs>
     </Member>
     <Member MemberName="AspectFit">
@@ -80,6 +78,7 @@
       </ReturnValue>
       <Docs>
         <summary>Scale the image so it exactly fills the view. Scaling may not be uniform in X and Y.</summary>
+        <remarks>To be added.</remarks>
       </Docs>
     </Member>
   </Members>