Move Unix Time docs to instant type

Instead of only specifying the epoch of the instant returned from now(),
specify the epoch of the instant type. It then follows that now()
returns an instant with that epoch.

Author: ptomato

wit-0.3.0-draft/system-clock.wit

@@ -14,12 +14,19 @@ interface system-clock {
     /// An "instant", or "exact time", is a point in time without regard to any
     /// time zone: just the time since a particular external reference point,
     /// often called an "epoch".
+    ///
+    /// Here, the epoch is 1970-01-01T00:00:00Z, also known as
+    /// [POSIX's Seconds Since the Epoch], also known as [Unix Time].
+    ///
     /// Note that even if the seconds field is negative, incrementing
     /// nanoseconds always represents moving forwards in time.
     /// For example, `{ -1 seconds, 999999999 nanoseconds }` represents the
     /// instant one nanosecond before the epoch.
     /// For more on various different ways to represent time, see
     /// https://tc39.es/proposal-temporal/docs/timezone.html
+    ///
+    /// [POSIX's Seconds Since the Epoch]: https://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xbd_chap04.html#tag_21_04_16
+    /// [Unix Time]: https://en.wikipedia.org/wiki/Unix_time
     @since(version = 0.3.0)
     record instant {
         seconds: s64,
@@ -38,14 +45,7 @@ interface system-clock {
     /// This clock is not monotonic, therefore calling this function repeatedly
     /// will not necessarily produce a sequence of non-decreasing values.
     ///
-    /// The returned timestamps represent the number of seconds since
-    /// 1970-01-01T00:00:00Z, also known as [POSIX's Seconds Since the Epoch],
-    /// also known as [Unix Time].
-    ///
     /// The nanoseconds field of the output is always less than 1000000000.
-    ///
-    /// [POSIX's Seconds Since the Epoch]: https://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xbd_chap04.html#tag_21_04_16
-    /// [Unix Time]: https://en.wikipedia.org/wiki/Unix_time
     @since(version = 0.3.0)
     now: func() -> instant;