Document optforfuzzing attribute created in r328214.

llvm-svn: 328236
diff --git a/llvm/docs/BitCodeFormat.rst b/llvm/docs/BitCodeFormat.rst
index 24c007a..6da69bc 100644
--- a/llvm/docs/BitCodeFormat.rst
+++ b/llvm/docs/BitCodeFormat.rst
@@ -1055,6 +1055,7 @@
 * code 54: ``strictfp``
 * code 55: ``sanitize_hwaddress``
 * code 56: ``nocf_check``
+* code 57: ``optforfuzzing``
 
 .. note::
   The ``allocsize`` attribute has a special encoding for its arguments. Its two