Add missing alt attributes in publican docs.

Haven't figured out how to regenerate the html, so it's not tested.
Looks like remaining html errors will be fixed in the next publican
release:  https://bugzilla.redhat.com/show_bug.cgi?id=788576

doc/Wayland/en_US/Architecture.xml

@@ -20,6 +20,11 @@
       <imageobject>
 	<imagedata fileref="images/x-architecture.png" format="PNG" />
       </imageobject>
+      <textobject>
+        <phrase>
+          X architecture diagram
+        </phrase>
+      </textobject>
     </mediaobject>
     <para>
       <orderedlist>
@@ -124,6 +129,11 @@
       <imageobject>
 	<imagedata fileref="images/wayland-architecture.png" format="PNG" />
       </imageobject>
+      <textobject>
+        <phrase>
+          Wayland architecture diagram
+        </phrase>
+      </textobject>
     </mediaobject>
     <para>
       <orderedlist>

doc/Wayland/en_US/Book_Info.xml

@@ -27,6 +27,11 @@
       <imageobject>
 	<imagedata fileref="images/wayland.png" format="PNG" />
       </imageobject>
+      <textobject>
+        <phrase>
+          Wayland logo
+        </phrase>
+      </textobject>
     </inlinemediaobject>
   </corpauthor>
   <xi:include href="Common_Content/Legal_Notice.xml" xmlns:xi="http://www.w3.org/2001/XInclude" />

doc/Wayland/en_US/Overview.xml

@@ -45,6 +45,11 @@
       <imageobject>
 	<imagedata fileref="images/x-architecture.png" format="PNG" />
       </imageobject>
+      <textobject>
+        <phrase>
+          X architecture diagram
+        </phrase>
+      </textobject>
     </mediaobject>
     <para>
       The diagram above illustrates the central role of the X server and
@@ -103,6 +108,11 @@
       <imageobject>
 	<imagedata fileref="images/wayland-architecture.png" format="PNG" />
       </imageobject>
+      <textobject>
+        <phrase>
+          Wayland architecture diagram
+        </phrase>
+      </textobject>
     </mediaobject>
     <para>
       The figure above illustrates how Wayland clients interact with a