Add commit query param docs to create transaction call

[DanWebb]

As discussed here a "commit" query string parameter can be added to the "create transaction" api call in order to create and commit a transaction in one step.

This was certainly helpful for my use case and wouldn't have known about unless Jorge mentioned it so feel it should be documented somewhere.

[fixe]

Thank you, landed in https://github.com/uphold/docs/commit/0571143ba8cb478682e3be8f00a14c3fb45e91e4.