Bug 27058: (follow-up) Clarify notforloan checks with a comment

Bug 27058: (follow-up) Clarify notforloan checks with a comment

In IsAvailableForItemLevelRequest the check is for holdability and in
ItemsAnyAvailableAndNotRestricted the check is for
checkoutability. These comments should make it more clear because the
notforloan value is used for these two different purposes and is a bit
confusing (we might want to add a new field "notforhold" in future to
make the code self documenting)

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