spec: Allow <annotation> in <arg> elements in introspection XML

This is widely used in practice (especially by GLib — just look at files
in /usr/share/dbus-1/interfaces/), and there is no reason not to allow
it. Update the specification, introspection DTD and XSL file to allow
and represent it.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=86162
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>

doc/dbus-specification.xml

@@ -4045,7 +4045,7 @@
       </itemizedlist>
     </para>
     <para>
-        Method, interface, property, and signal elements may have
+        Method, interface, property, signal, and argument elements may have
         "annotations", which are generic key/value pairs of metadata.
         They are similar conceptually to Java's annotations and C# attributes.
         Well-known annotations:

doc/introspect.dtd

@@ -16,7 +16,7 @@
 <!ELEMENT signal (arg|annotation)*>
 <!ATTLIST signal name CDATA #REQUIRED>
 
-<!ELEMENT arg EMPTY>
+<!ELEMENT arg (annotation)*>
 <!ATTLIST arg name CDATA #IMPLIED>
 <!ATTLIST arg type CDATA #REQUIRED>
 <!-- Method arguments SHOULD include "direction",

doc/introspect.xsl

@@ -80,6 +80,12 @@
                       
                       <span class="type"><xsl:value-of select="@type"/></span><xsl:text> </xsl:text>
                       <span class="symbol"><xsl:value-of select="@name"/></span><xsl:text> </xsl:text>
+
+                      <xsl:if test="annotation">
+                        <ul>
+                          <xsl:apply-templates select="annotation"/>
+                        </ul>
+                      </xsl:if>
                     </li>
                   </xsl:for-each>
                 </ul>