Bug 21896: (QA follow-up) Document and Test for FIFO behaviour

Bug 21896: (QA follow-up) Document and Test for FIFO behaviour

A short dicussion lead to the decision to make it explictly clear that
this method will implicitly apply credits against debits in a 'First In
First Out' manor, meaning oldest outstanding debits will be paid off
first.

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>
Signed-off-by: Nick Clemens <nick@bywatersolutions.com>