Bug 20544: Wrong comment database documentation on itemnotes

Bug 20544: Wrong comment database documentation on itemnotes

Fixes the comments in kohastructure.sql to state the right
MARC21 fields for itemnotes (public and internal).

To test:
- Create a new Koha database and make sure it still installs
- Read the patch and check comments are correct

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
(cherry picked from commit 60347dcc0724ad354cfabb89ca024dfa269e62e5)
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>