snippets manual is not rebuilt

[Federico Bruni]

I'm translating the snippets manual for the first time and I see that 
`make doc` does not rebuild it when source is updated.
It's a bug or I'm missing something?
How can I force it?

NOTE: As a translator I do _not_ compile out of tree.


$ ls -l out-www/offline-root/Documentation/snippets/index*
-rw-rw-r--. 1 fede fede 101600 3 dic 23.25 
out-www/offline-root/Documentation/snippets/index.de.html
-rw-rw-r--. 1 fede fede 112835 3 dic 23.25 
out-www/offline-root/Documentation/snippets/index.es.html
-rw-rw-r--. 1 fede fede 111393 3 dic 23.25 
out-www/offline-root/Documentation/snippets/index.fr.html
-rw-rw-r--. 1 fede fede 129586 3 dic 23.25 
out-www/offline-root/Documentation/snippets/index.html
-rw-rw-r--. 1 fede fede 103948 3 dic 23.25 
out-www/offline-root/Documentation/snippets/index.it.html
-rw-rw-r--. 1 fede fede 171102 3 dic 23.25 
out-www/offline-root/Documentation/snippets/index.ja.html
-rw-rw-r--. 1 fede fede 187593 3 dic 23.25 
out-www/offline-root/Documentation/snippets/index_toc.de.html
-rw-rw-r--. 1 fede fede 210040 3 dic 23.25 
out-www/offline-root/Documentation/snippets/index_toc.es.html
-rw-rw-r--. 1 fede fede 204845 3 dic 23.25 
out-www/offline-root/Documentation/snippets/index_toc.fr.html
-rw-rw-r--. 1 fede fede 244000 3 dic 23.25 
out-www/offline-root/Documentation/snippets/index_toc.html
-rw-rw-r--. 1 fede fede 192694 3 dic 23.25 
out-www/offline-root/Documentation/snippets/index_toc.it.html
-rw-rw-r--. 1 fede fede 326273 3 dic 23.25 
out-www/offline-root/Documentation/snippets/index_toc.ja.html

$ ls -l Documentation/it/snippets.tely
-rw-rw-r--. 1 fede fede 4273 8 dic 23.20 Documentation/it/snippets.tely