add missing stopwords

Keep the spelling tests happy.
state minimum versions for new features
reference Authorisation Only transactions in docs
document credit card account number examples

Reword account number input format requirement.
allow Transaction objects to have no code

[rt.cpan.org #93739] shows that some account types lack a transaction
code for Authorisation Only transactions.
v1.2.6

  - Allow an undefined transaction code in Transaction objects, since
    Bankwest does not return one in some cases ([rt.cpan.org #93739]).
    Thanks to Owen Hollands for reporting this behaviour and providing
    a patch.

  - Minor documentation adjustments.