Specify additional headers that don't have a convenience header.

asio/src/doc/reference.xsl

diff --git a/asio/src/doc/reference.xsl b/asio/src/doc/reference.xsl
index e41d34e..98279f9 100644
--- a/asio/src/doc/reference.xsl
+++ b/asio/src/doc/reference.xsl
@@ -752,12 +752,18 @@
     <xsl:when test="contains($file, 'include/asio/high_resolution_timer')">
       <xsl:text>None</xsl:text>
     </xsl:when>
+    <xsl:when test="contains($file, 'include/asio/spawn')">
+      <xsl:text>None</xsl:text>
+    </xsl:when>
     <xsl:when test="contains($file, 'include/asio/steady_timer')">
       <xsl:text>None</xsl:text>
     </xsl:when>
     <xsl:when test="contains($file, 'include/asio/system_timer')">
       <xsl:text>None</xsl:text>
     </xsl:when>
+    <xsl:when test="contains($file, 'include/asio/use_future')">
+      <xsl:text>None</xsl:text>
+    </xsl:when>
     <xsl:otherwise>
       <xsl:text>[^asio.hpp]</xsl:text>
     </xsl:otherwise>