Bug 22752: (follow-up) Add style; Only show message about item-level holds

Bug 22752: (follow-up) Add style; Only show message about item-level holds

I think it's generally assumed that a hold has, by default, been made at
the bibliographic level. I think it's sufficent to add a note for only
item-level holds.

To test, apply the patch and log into the OPAC as a user who has
multiple holds, some at the bibliographic level and some at the item
level.

- Go to Your summary --> Holds
- Confirm that any item-level hold has a note under the title: "Item on
  hold <barcode>".
- The note should be on a new line and styled differently than the other
  text.

Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>