doc: add the missing 'handle' attribute in xml
The XML response to the "qXfer:threads:read" packet may include
a "handle" attribute.  The attribute is mentioned in the document
but not shown in the sample XML structure.  Add it.
Reviewed-By: Eli Zaretskii <eliz@gnu.org>
diff --git a/gdb/doc/gdb.texinfo b/gdb/doc/gdb.texinfo
index 727f927..82a617e 100644
--- a/gdb/doc/gdb.texinfo
+++ b/gdb/doc/gdb.texinfo
@@ -48081,7 +48081,7 @@
 @smallexample
 <?xml version="1.0"?>
 <threads>
-    <thread id="id" core="0" name="name">
+    <thread id="id" core="0" name="name" handle="1a2b3c">
     ... description ...
     </thread>
 </threads>