[client] Explain that transactions alerts are based on absolute amount

The current alerts are really misleading IMO: the amount compared to the threshold is forced to an absolute value. So -19 and 19 are both greater than 10 for example.

See https://framagit.org/kresusapp/kresus/-/blob/master/server/models/entities/alerts.ts#L63

We spoke about revamping the whole alert system so I did not modify the behavior and just tweaked the explanation. I did not modify the labels either, fearing that they would become too large.