Add comment about what -N -l does.

This was added in aosp/1642084 and I forgot to address a comment.

Test: "m nothing".
Change-Id: I0fd2f2e8eeaa5b316cf19c3b426f708f2587e60f
diff --git a/bootstrap/config.go b/bootstrap/config.go
index 5cd1712..6be0111 100644
--- a/bootstrap/config.go
+++ b/bootstrap/config.go
@@ -67,6 +67,7 @@
 	})
 	debugFlagsVariable = bootstrapVariable("debugFlags", func(c BootstrapConfig) string {
 		if c.DebugCompilation() {
+			// -N: disable optimizations, -l: disable inlining
 			return "-N -l"
 		} else {
 			return ""