Add fast type specific promotion note

Author: mborland

doc/modules/ROOT/pages/fast_types.adoc

@@ -210,7 +210,8 @@ decimal_fast128_t d);
 === 3.2.8 Note
 In the event of binary arithmetic between a non-decimal type and a decimal type the arithmetic will occur between the native types, and the result will be returned as the same type as the decimal operand. (e.g. decimal_fast32_t * uint64_t -> decimal_fast32_t)
 
-In the event of binary arithmetic between two decimal types the result will be the higher precision type of the two (e.g. decimal64_t + decimal_fast32_t -> decimal64_t)
+In the event of binary arithmetic between two decimal types the result will be the higher precision type of the two (e.g. decimal64_t + decimal_fast32_t -> decimal64_t).
+If the binary arithmetic is between an IEEE type, and it's fast equivalent, the result is the fast type (e.g. decimal_fast128_t - decimal128_t -> decimal_fast128_t).
 
 
 [#fast_operator_behavior]