Make PaintOptions example a better role model

Do not have members that are both optional and nullable.

Fixes #1204.

Author: mgiuca

index.bs

@@ -5036,8 +5036,8 @@ No [=extended attributes=] are applicable to dictionaries.
         };
 
         dictionary PaintOptions {
-          DOMString? fillPattern = "black";
-          DOMString? strokePattern = null;
+          DOMString fillPattern = "black";
+          DOMString strokePattern;
           Point position;
         };
 
@@ -5059,10 +5059,12 @@ No [=extended attributes=] are applicable to dictionaries.
         ctx.drawRectangle(300, 200, { fillPattern: "red", position: new Point(10, 10) });
     </pre>
 
-    Both fillPattern and strokePattern are given [=dictionary member/default values=],
-    so if they are omitted, the definition of drawRectangle can assume that they
-    have the given default values and not include explicit wording to handle
-    their non-presence.
+    The members of <code class="idl">PaintOptions</code> are [=dictionary member/optional=].
+    If <code class="idl">fillPattern</code> is omitted, the definition of
+    <code class="idl">drawRectangle</code> can assume that it has the given default values
+    and not include explicit wording to handle its omission.
+    <code class="idl">drawRectangle</code> needs to explicitly handle the case where
+    <code class="idl">strokePattern</code> and <code class="idl">position</code> are omitted.
 
 </div>