Add subtraction behavior

Author: mborland

doc/modules/ROOT/pages/generic_decimal.adoc

@@ -212,3 +212,26 @@ Returns the result of `lhs + rhs` subject to:
 . If either `lhs` or `rhs` are `INF`, returns `INF` of same sign
 . If `lhs + rhs` overflows, returns `INF`
 . If `lhs + rhs` underflows, returns `0`
+
+=== Subtraction
+
+[source,c++]
+----
+// 3.2.8 binary arithmetic operators:
+// LHS and RHS can be any integer or decimal type
+
+constexpr /* see 3.2.8 */ operator-(LHS lhs, decimal32_t rhs) noexcept;
+constexpr /* see 3.2.8 */ operator-(LHS lhs, decimal64_t rhs) noexcept;
+constexpr /* see 3.2.8 */ operator-(LHS lhs, decimal128_t rhs) noexcept;
+constexpr /* see 3.2.8 */ operator-(decimal32_t lhs, RHS rhs) noexcept;
+constexpr /* see 3.2.8 */ operator-(decimal64_t lhs, RHS rhs) noexcept;
+constexpr /* see 3.2.8 */ operator-(decimal128_t lhs, RHS rhs) noexcept;
+----
+
+Returns the result of `lhs - rhs` subject to:
+
+. If either `lhs` or `rhs` are `NAN`, returns `QNAN`
+. If `lhs` and `rhs` are `INF` of opposite sign, returns `QNAN`
+. If either `lhs` or `rhs` are `INF`, returns `INF` of same sign
+. If `lhs - rhs` overflows, returns `INF`
+. If `lhs - rhs` underflows, returns `0`