Note about why this isn't vendor_available.

Test: None
Bug: None
Change-Id: I0c37a344dfb542000424615dfbead3ff7eab8aab

Android.bp

@@ -149,6 +149,9 @@ cc_library_static {
     ],
 }
 
+// Not available to vendor modules until libc++ is updated and this library is
+// merged into libc++ proper.
+// https://issuetracker.google.com/147469372
 cc_library_static {
     name: "libc++fs",
     recovery_available: true,