Index: docs/LangRef.rst
===================================================================
--- docs/LangRef.rst
+++ docs/LangRef.rst
@@ -7567,10 +7567,13 @@
 
 The '``fptrunc``' instruction casts a ``value`` from a larger
 :ref:`floating point <t_floating>` type to a smaller :ref:`floating
-point <t_floating>` type. If the value cannot fit (i.e. overflows) within the
-destination type, ``ty2``, then the results are undefined. If the cast produces
-an inexact result, how rounding is performed (e.g. truncation, also known as
-round to zero) is undefined.
+point <t_floating>` type. If the real number (``R``) represented by ``value`` cannot be
+exactly represented using the :ref:`floating point <t_floating>` type ``ty2``, then the
+value returned must be either ``R`` truncated to fit in ``ty2`` (i.e.  round to
+zero), or the successor to ``R`` in ``ty2`` (i.e. round up), or the predecesor
+to ``R`` (i.e. round down) in ``ty2``, or any of the special values in ``ty2``
+(i.e. +/- infinity, +/- zero and NaN). The method for choosing which of these
+values to return is undefined.
 
 Example:
 """"""""