[LangRef] add note about format of FP types

llvm-svn: 328105
diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst
index dce25c5..55b9621 100644
--- a/llvm/docs/LangRef.rst
+++ b/llvm/docs/LangRef.rst
@@ -2577,6 +2577,10 @@
    * - ``ppc_fp128``
      - 128-bit floating-point value (two 64-bits)
 
+The binary format of half, float, double, and fp128 correspond to the
+IEEE-754-2008 specifications for binary16, binary32, binary64, and binary128
+respectively.
+
 X86_mmx Type
 """"""""""""