pkg/platform: move package doc to platform-agnostic file

Make sure the package's documentation is available for any platform,
not just "unix".

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

pkg/platform/platform.go

@@ -1,3 +1,5 @@
+// Package platform provides helper function to get the runtime architecture
+// for different platforms.
 package platform // import "github.com/docker/docker/pkg/platform"
 
 import (

pkg/platform/platform_unix.go

@@ -1,8 +1,6 @@
 //go:build !windows
 // +build !windows
 
-// Package platform provides helper function to get the runtime architecture
-// for different platforms.
 package platform // import "github.com/docker/docker/pkg/platform"
 
 import (