Minor changes on Section 12.2.3

For better readability:

• from "An implementation event represents a state change related to implementation or migration." to "An implementation event represents something that happens or occurs with the implementation process, especially something significant or noteworthy, such as a state change related to implementation or migration."
• from "Work packages may be triggered or interrupted by an implementation event. Also, work packages may raise events that trigger other behavior." to "An implementation event may trigger or be triggered (raised) by a work package or a plateau. An implementation event may access a deliverable and may be composed of other implementation events."
• from "Unlike a work package, an event is instantaneous: it does not have a duration." to "Unlike a work package, an implementation event is instantaneous: it does not have a duration."
• from "An implementation event may have a time attribute that denotes the moment or moments at which the event happens." to "An implementation event may have a time attribute that denotes the moment or moments at which the event happens, or its frequency."