add more docs on *nan

diff --git a/cgen/doc/rtl.texi b/cgen/doc/rtl.texi
index 0b69b1c..430c81d 100644 (file)
--- a/cgen/doc/rtl.texi
+++ b/cgen/doc/rtl.texi
@@ -2789,6 +2789,7 @@ Perform a mathematical operation.
 
 @item (*nan mode operand)
 Return a boolean indicating if @samp{operand} is a NaN.
+@samp{mode} must be a floating point mode.
 There are three versions.
 
 @itemize @bullet