clarify NaN propagation in fptrunc (#68554)

Follow-up to #66579: while
implementing those semantics in Miri I realized there's a special case
to be considered in truncating float casts.

llvm/docs/LangRef.rst

@@ -11332,7 +11332,10 @@ environment <floatenv>`.
 NaN values follow the usual :ref:`NaN behaviors <floatnan>`, except that _if_ a
 NaN payload is propagated from the input ("Quieting NaN propagation" or
 "Unchanged NaN propagation" cases), then the low order bits of the NaN payload
-which cannot fit in the resulting type are discarded.
+which cannot fit in the resulting type are discarded. Note that if discarding
+the low order bits leads to an all-0 payload, this cannot be represented as a
+signaling NaN (it would represent an infinity instead), so in that case
+"Unchanged NaN propagation" is not possible.
 
 Example:
 """"""""