Provide an info about pspReference limit in docs for transaction API

[korycins]

The section that should menton about limit of 512 characters for pspRefrence should be:

• https://docs.saleor.io/docs/3.x/developer/payments#creating-transactions - the mutation will raise an exception
• https://docs.saleor.io/docs/3.x/developer/payments#updating-transactions - the mutation will raise an exception
• All responses from chapter: https://docs.saleor.io/docs/3.x/developer/extending/webhooks/synchronous-events/transaction - the failure TransactionEvent with proper message will be created.

Core PR related to this issue: https://github.com/saleor/saleor/pull/13608