lint: avoid revive package-comments rule on observer.go

The comment block above 'package shutdown' in observer.go was
treated as a package doc comment, which revive requires to start
with 'Package shutdown ...'. doc.go already provides that canonical
package doc.

Move the fan-out rationale below the package declaration as a
regular block comment — same documentation value, no revive
collision.

Author: khanakia

observer.go

@@ -1,17 +1,18 @@
-// observer.go — synchronous fan-out helpers for the Observer pattern.
-//
-// All fire* methods take a snapshot of the observer slice under the
-// manager's read lock before invoking, so a Subscribe call concurrent
-// with shutdown can't cause a torn iteration. Observers fire
-// synchronously: a slow observer slows shutdown — adapters that need
-// async work should fan out to a goroutine themselves.
 package shutdown
 
 import (
 	"os"
 	"time"
 )
 
+// Observer fan-out helpers.
+//
+// All fire* methods take a snapshot of the observer slice under the
+// manager's read lock before invoking, so a Subscribe call concurrent
+// with shutdown can't cause a torn iteration. Observers fire
+// synchronously: a slow observer slows shutdown — adapters that need
+// async work should fan out to a goroutine themselves.
+
 // fireOnSignal invokes each observer's OnSignal hook.
 func (m *Manager) fireOnSignal(sig os.Signal) {
 	m.mu.RLock()