Explain why RTLD_LAZY isn't supported.

Change-Id: Ib2377d50acbda6fd3eade2bba4bfb50e759dd1f9
diff --git a/libc/include/dlfcn.h b/libc/include/dlfcn.h
index a90c4f8..d65a409 100644
--- a/libc/include/dlfcn.h
+++ b/libc/include/dlfcn.h
@@ -146,7 +146,11 @@
  */
 #define RTLD_LOCAL    0
 
-/** Not supported on Android; Android always uses RTLD_NOW. */
+/**
+ * Not supported on Android. Android always uses RTLD_NOW for security reasons.
+ * Resolving all undefined symbols before dlopen() returns means that RELRO
+ * protections can be applied to the PLT before dlopen() returns.
+ */
 #define RTLD_LAZY     0x00001
 
 /** A dlopen() flag to resolve all undefined symbols before dlopen() returns. */