[GNC] Invoices' internal note is not reported in ledger

[Giuseppe Foti]

   Hi, I have noticed invoices internal notes are not reported in the
   general ledger (showed on 2 lines).

   It seems is not possible to insert a note in the ledger for
   transactions generated by invoices/bills, and this makes it impossible
   to generate a filtered view using notes as a tag or similar.

   I can insert notes (shown in the ledger) for any other transaction.

   Is there something I'm missing?