Update snapshots of rustdoc tests to take into account the comment highlighting

Author: GuillaumeGomez

tests/rustdoc/where.SWhere_Simd_item-decl.html

@@ -1,3 +1,3 @@
-<pre class="rust item-decl"><code>pub struct Simd&lt;T&gt;(/* private fields */)
+<pre class="rust item-decl"><code>pub struct Simd&lt;T&gt;(<span class="comment">/* private fields */</span>)
 <span class="where">where
-    T: <a class="trait" href="trait.MyTrait.html" title="trait foo::MyTrait">MyTrait</a></span>;</code></pre>
+    T: <a class="trait" href="trait.MyTrait.html" title="trait foo::MyTrait">MyTrait</a></span>;</code></pre>

tests/rustdoc/where.alpha_trait_decl.html

@@ -1,3 +1,3 @@
-<code>pub struct Alpha&lt;A&gt;(/* private fields */)
+<code>pub struct Alpha&lt;A&gt;(<span class="comment">/* private fields */</span>)
 <span class="where">where
     A: <a class="trait" href="trait.MyTrait.html" title="trait foo::MyTrait">MyTrait</a></span>;</code>

tests/rustdoc/whitespace-after-where-clause.union.html

@@ -1,4 +1,4 @@
 <pre class="rust item-decl"><code>pub union Union&lt;'a, B&gt;<span class="where fmt-newline">where
     B: <a class="trait" href="trait.ToOwned.html" title="trait foo::ToOwned">ToOwned</a>&lt;<a class="primitive" href="{{channel}}/std/primitive.unit.html">()</a>&gt; + ?<a class="trait" href="{{channel}}/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a> + 'a,</span>{
-    /* private fields */
+    <span class="comment">/* private fields */</span>
 }</code></pre>

tests/rustdoc/whitespace-after-where-clause.union2.html

@@ -1,3 +1,3 @@
 <pre class="rust item-decl"><code>pub union Union2&lt;'a, B: ?<a class="trait" href="{{channel}}/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a> + <a class="trait" href="trait.ToOwned.html" title="trait foo::ToOwned">ToOwned</a>&lt;<a class="primitive" href="{{channel}}/std/primitive.unit.html">()</a>&gt; + 'a&gt; {
-    /* private fields */
+    <span class="comment">/* private fields */</span>
 }</code></pre>