Change 3.1 Timestamp definition (merge section two into this)

[wtoorop]

Mention the definition is overloaded and make clear with what we mean with it.

[KTeichel]

How does this issue relate to the definitions I proposed in issue #16 ? Does my proposal there solve the second part of this issue?

[partim]

The definition there defines timestamp to be a clock value of any clock while we (perhaps implicitly) always meant them to refer to the real-time clock.

I’m not sure how to deal with this – our ultimate advise is to use raw clock timestamps for time spans. That is one way of doing this: Instead of saying "don’t use timestamps", say "consider the clock you are using for your timestamps." The other is to define a new term. At this time, I think I prefer the former.