Update pay endpoint doc to include optional MerchantInitiatedReason

[philmeehan]

What does this PR do? (please provide any background)

This updates the docs for the payment gateway api to show that we support an optional merchant initiated reason.

We will be asking Firemelon to send this down to us for Annual Multi Trips when the policy is created (the payment service will then work out how to get the right keys down to Worldpay dependent on what it already has in the database.

My actual commit before automation kicked in c5c286b

Any tech debt?

No

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