libbinder: document no_apex

With alternative and also short explanation, since people have asked.

Bug: 139016109
Test: N/A
Change-Id: I5562cf9e719d69c081bfbb79d3c4b394e3994f56
diff --git a/libs/binder/Android.bp b/libs/binder/Android.bp
index c4aed95..2da9f3f 100644
--- a/libs/binder/Android.bp
+++ b/libs/binder/Android.bp
@@ -37,6 +37,10 @@
         enabled: true,
     },
     double_loadable: true,
+
+    // libbinder does not offer a stable wire protocol.
+    // if a second copy of it is installed, then it may break after security
+    // or dessert updates. Instead, apex users should use libbinder_ndk.
     no_apex: true,
 
     srcs: [