An alternative to @mgrol's correct answer, which makes it much easier to control the 'wrap width' of notes is the skinparam wrapWidth parameter - it works like this:
(Click through to see the parameter in context and change the values)
It is not something that breaks in Macros, as is shown in the example - so it can be used commonly. Also, it's worth mentioning the maxMessageSize parameter as well - which allows similar control of the messages.
skinparam wrapWidth 30
skinparam maxMessageSize 30
/' Macros '/
!definelong Event(from, to, message, notes)
from ->> to : message
'/ End Macros/'
participant "Event Stream" as K #LightGreen
participant "A Service" as SRV #Green
Event(K, SRV, "This is a long message", "This could be a multiline text, but it is not.")
K ->> SRV: non-macro long message
note right: Here is some more text that could cause a problem and should be wrapped by the wrapWidth skinaparam.