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I totally agree, this is missing...
It can be something simple like this:
A |->A: message text.
Or global configuration to always use left side for last actor in diagram,
This will avoid having text going far right side.
http://www.plantuml.com/plantuml/png/SoWkIImgAStDuNBAJrBGjLDmpCbCJbMmKiX8pSd9vmBpC8GLF1Ko4ijI5H8LSl9B54ho5OeJAnMAynLoKaiKSbCLm4RgwUafnEGvvIGg866O9I3AIi4t5QWtNmUhQTYrEwJcfG3T2W00
With last beta http://beta.plantuml.net/plantuml.jar you can now have :
@startuml Bob -> Alice : hello Alice <- Alice : hello I have a lot to say so let me start... \nBla Bla Bla Bla @enduml
This is not widely tested...Is this what you were expecting ?