XEP-0030: Add the disco#info feature in every example.

Section 3.1 says “Every entity MUST […] support at least the
'http://jabber.org/protocol/disco#info' feature”, so it makes sense to
show that in examples.

Thanks to George Kiagiadakis for having pointed that out, at
https://bugs.freedesktop.org/show_bug.cgi?id=98127#c5

xep-0030.xml

@@ -337,6 +337,7 @@
     id='info2'>
   <query xmlns='http://jabber.org/protocol/disco#info'>
     <identity category='account' type='registered'/>
+    <feature var='http://jabber.org/protocol/disco#info'/>
   </query>
 </iq>
       ]]></example>
@@ -358,6 +359,7 @@
         category='conference'
         name='A Dark Cave'
         type='text'/>
+    <feature var='http://jabber.org/protocol/disco#info'/>
     <feature var='http://jabber.org/protocol/muc'/>
     <feature var='muc_passwordprotected'/>
     <feature var='muc_hidden'/>
@@ -385,6 +387,7 @@
         category='client'
         type='pc'
         name='Gabber'/>
+    <feature var='http://jabber.org/protocol/disco#info'/>
     <feature var='jabber:iq:time'/>
     <feature var='jabber:iq:version'/>
   </query>
@@ -413,6 +416,7 @@
     <identity
         category='automation'
         type='command-list'/>
+    <feature var='http://jabber.org/protocol/disco#info'/>
   </query>
 </iq>
       ]]></example>