So far I have adopted the convention of using a container drawn in a different colour. This is sufficient providing that one large caveat remains true - there are no overlapping transactions, only flat or nested transactions. Generally speaking for good design and robust transaction isolation this would be true. However, it may be better to adopt a notation that allows a single State to exist within more than one transaction. There may be another objection to my existing notation, that it simply introduces yet another box or container, and however, this is rendered, using a different colour, dotted line, filled background etc.
Alternatives are possible.
For example, taking a cue from the UML Class notation for an interface a lollipop might be added to each state indicating the name of any transactions that it belongs to. For the time being, I am sticking with the proposal that transactions be indicated using a simple box as shown below and that the transaction be named in the upper right corner for identification purposes. Additionally, I would like to modify the notation to clearly mark the starting point of a transaction within the transaction boundary. I suggest that we do this by adopting the stereotype notation from other UML notations into the Statechart notation. Stereotypes would be a generally useful addition to the notation in any case.