doc: tweak definition of Timestamp

The previous definition was slightly incorrect, or
at best, imprecise. It is actually pretty important
to mention that `Timestamp` is in the Unix time
scale (the same as the POSIX time scale), as this
clarifies that it behaves as if leap seconds don't
exist.

PR #134

src/timestamp.rs

@@ -23,7 +23,7 @@ use crate::{
 /// An instant in time represented as the number of nanoseconds since the Unix
 /// epoch.
 ///
-/// A timestamp is always in UTC.
+/// A timestamp is always in the Unix timescale with a UTC offset of zero.
 ///
 /// To obtain civil or "local" datetime units like year, month, day or hour, a
 /// timestamp needs to be combined with a [`TimeZone`] to create a [`Zoned`].