[flang] documentation update

Author: vdonaldson

flang/docs/Extensions.md

@@ -815,6 +815,14 @@ print *, [(j,j=1,10)]
   integer overflow never occurs in address calculations and increment of
   do-variable unless the option `-fwrapv` is enabled.
 
+* Two new ieee_round_type values were added in f18 beyond the four values
+  defined in f03 and f08: ieee_away and ieee_other. Contemporary hardware
+  typically does not have support for these rounding modes;
+  ieee_support_rounding calls for these values return false.
+  ieee_set_rounding_mode calls that attempt to set the rounding mode to one
+  of these values in violation of the restriction in f23 clause 17.11.42 set
+  the mode to ieee_nearest.
+
 ## De Facto Standard Features
 
 * `EXTENDS_TYPE_OF()` returns `.TRUE.` if both of its arguments have the