Prefix woven definition with text indicator

This helps make clear that the code snippet is not an example, or part of an
example that ends the description (in the docblock).

src/texinfo.xsl

@@ -143,6 +143,8 @@
                       $xt:nl,
                       $doc,
                       $xt:nl,
+                      '@emph{Definition:}',
+                      $xt:nl,
                       '@verbatim',
                       $xt:nl,
                       xt:serialize( . ),