clarify that unsafeCoerce# :: Float# -> Int# is not safe (see #2209)

compiler/prelude/primops.txt.pp

@@ -1801,6 +1801,7 @@ pseudoop   "unsafeCoerce#"
          * Casting {\tt Any} back to the real type
 
          * Casting an unboxed type to another unboxed type of the same size
+           (but not coercions between floating-point and integral types)
 
          * Casting between two types that have the same runtime representation.  One case is when
            the two types differ only in "phantom" type parameters, for example