Clarify reason for native_coverage being off for libdl

Bug: http://b/124067925

It's off because it doesn't link with any system_shared_libs.

Test: N/A
Change-Id: I293fb9ba8df213f8f6bbec85135a974e5966f0cc
diff --git a/libdl/Android.bp b/libdl/Android.bp
index 43ddbfe..642cc7a 100644
--- a/libdl/Android.bp
+++ b/libdl/Android.bp
@@ -92,6 +92,8 @@
 
     nocrt: true,
     system_shared_libs: [],
+
+    // Opt out of native_coverage when opting out of system_shared_libs
     native_coverage: false,
 
     // This is placeholder library the actual implementation is (currently)