docs(wit): document capability and component contracts

wit/clock/package.wit

@@ -1,9 +1,16 @@
 package wasmeld:clock@0.1.0;
 
+/// Actor-local monotonic time for measuring elapsed durations.
 interface monotonic-clock {
+  /// Returns nanoseconds elapsed since this Actor Store was created.
+  ///
+  /// The value is monotonic and saturates at `u64::MAX`. It is not Unix time,
+  /// cannot be compared across Actor restarts, and must not be persisted as a
+  /// wall-clock timestamp.
   now: func() -> u64;
 }
 
+/// Host-side world used by Wasmeld to link the clock implementation.
 world clock-host {
   import monotonic-clock;
 }