Add documentation for target-sdk-version flag

This is using min_sdk_version and not target_sdk_version value from
Android.bp. Add documentation on why it has been implemented as such.

Test: N/A
Change-Id: I7da38ed351562dc530cde4c0d6bf7bd6b691f01d
diff --git a/java/aar.go b/java/aar.go
index a483e13..068a9c7 100644
--- a/java/aar.go
+++ b/java/aar.go
@@ -226,6 +226,8 @@
 	}
 
 	linkFlags = append(linkFlags, "--min-sdk-version "+minSdkVersion)
+	// Use minSdkVersion for target-sdk-version, even if `target_sdk_version` is set
+	// This behavior has been copied from Make.
 	linkFlags = append(linkFlags, "--target-sdk-version "+minSdkVersion)
 
 	// Version code