docs(wit): document capability and component contracts

wit/service/package.wit

@@ -1,11 +1,34 @@
 package wasmeld:service@0.1.0;
 
+/// Minimum request/response lifecycle implemented by every Wasmeld service.
+///
+/// The Runtime creates one Component Store per Actor, calls `init` once, then
+/// serializes `invoke` calls through that Actor's bounded mailbox. Component
+/// memory can therefore remain warm between calls, but it is not durable and
+/// is lost when the Actor or process restarts.
 world service-component {
+  /// Application-level failures returned without trapping the Component.
+  ///
+  /// Traps, fuel exhaustion, deadline interruption, and invalid ABI data are
+  /// Runtime failures and do not use this variant.
   variant service-error {
+    /// The Actor cannot accept calls because its initial configuration failed.
     init-failed(string),
+    /// This input could not be processed; a later call may still succeed.
     call-failed(string),
   }
 
+  /// Initializes a newly created Actor Store.
+  ///
+  /// `config` is opaque to the platform. Return `init-failed` for expected
+  /// configuration errors; do not retain borrowed views into this byte list.
   export init: func(config: list<u8>) -> result<_, service-error>;
+
+  /// Processes one opaque request and returns one opaque response.
+  ///
+  /// The management API represents these bytes as base64, while the public
+  /// gateway transports them as `application/octet-stream`. The Component
+  /// should return promptly and use the resident actor contract for long-lived
+  /// I/O instead of blocking this call in an internal loop.
   export invoke: func(input: list<u8>) -> result<list<u8>, service-error>;
 }