Bug 25508: Add info about caller responsabilities

We are removing the test on the existence of item and patron, callers
must call renewable before.

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

Koha/Account/Line.pm

@@ -912,7 +912,8 @@ sub renewable {
     my $renew_result = $line->renew_item;
 
 Conditionally attempt to renew an item and return the outcome. This is
-as a consequence of the fine on an item being fully paid off
+as a consequence of the fine on an item being fully paid off.
+Caller must call renewable before.
 
 =cut