Document missing conversion operators

Author: mborland

doc/modules/ROOT/pages/decimal_fast32_t.adoc

@@ -107,8 +107,11 @@ explicit constexpr operator std::float32_t() const noexcept;
 explicit constexpr operator std::float64_t() const noexcept;
 explicit constexpr operator std::bfloat16_t() const noexcept;
 
+explicit constexpr operator decimal32_t() const noexcept;
 explicit constexpr operator decimal64_t() const noexcept;
+explicit constexpr operator decimal_fast64_t() const noexcept;
 explicit constexpr operator decimal128_t() const noexcept;
+explicit constexpr operator decimal_fast128_t() const noexcept;
 
 }; // class decimal_fast32_t