Improve package level docs

Author: nhooyr

slog.go

@@ -4,7 +4,8 @@
 //
 // The examples are the best way to understand how to use this library effectively.
 //
-// Sink implementations are in the sloggers subdirectory.
+// This package only provides a high level API around Sink but does not provide any implementations.
+// The implementations are in the sloggers subdirectory.
 package slog // import "cdr.dev/slog"
 
 import (