Update documentation

doc/system/reference.adoc

@@ -622,6 +622,16 @@ namespace boost {
       friend bool operator!=( const std::error_code & lhs,
         const error_code & rhs ) noexcept;
 
+      template<class E>
+      friend constexpr bool operator==( const error_code & lhs, E rhs ) noexcept;
+      template<class E>
+      friend constexpr bool operator==( E lhs, const error_code & rhs ) noexcept;
+
+      template<class E>
+      friend constexpr bool operator!=( const error_code & lhs, E rhs ) noexcept;
+      template<class E>
+      friend constexpr bool operator!=( E lhs, const error_code & rhs ) noexcept;
+
       // conversions:
 
       operator std::error_code() const;
@@ -818,11 +828,11 @@ Returns: :: If `code` wraps a `std::error_code` object `ec`, `ec == static_cast<
   Otherwise, `code.category().equivalent( code.value(), condition ) || condition.category().equivalent( code, condition.value() )`.
 
 ```
-friend constexpr bool operator!=( const error_code & lhs,
+friend bool operator!=( const error_code & lhs,
   const error_condition & rhs ) noexcept;
 ```
 ```
-friend constexpr bool operator!=( const error_condition & lhs,
+friend bool operator!=( const error_condition & lhs,
   const error_code & rhs ) noexcept;
 ```
 [none]
@@ -862,6 +872,48 @@ friend bool operator!=( const std::error_code & lhs,
 Returns: ::
   `!( lhs == rhs )`.
 
+```
+template<class E>
+friend constexpr bool operator==( const error_code & lhs, E rhs ) noexcept;
+```
+[none]
+* {blank}
++
+Effects: ::
+[disc]
+** When `is_error_code_enum<E>::value` is `true`, returns `lhs == make_error_code(rhs)`;
+** When `is_error_condition_enum<E>::value` is `true`, returns `lhs == make_error_condition(rhs)`;
+** Otherwise, this overload is disabled.
+
+```
+template<class E>
+friend constexpr bool operator==( E lhs, const error_code & rhs ) noexcept;
+```
+[none]
+* {blank}
++
+Effects: ::
+[disc]
+** When `is_error_code_enum<E>::value` is `true`, returns `make_error_code(lhs) == rhs`;
+** When `is_error_condition_enum<E>::value` is `true`, returns `make_error_condition(lhs) == rhs`;
+** Otherwise, this overload is disabled.
+
+```
+template<class E>
+friend constexpr bool operator!=( const error_code & lhs, E rhs ) noexcept;
+```
+```
+template<class E>
+friend constexpr bool operator!=( E lhs, const error_code & rhs ) noexcept;
+```
+[none]
+* {blank}
++
+Returns: :: `!( lhs == rhs )`.
+Remarks: :: These overloads are only enabled when
+  `is_error_code_enum<E>::value` is `true` or
+  `is_error_condition_enum<E>::value` is `true`.
+
 #### Conversions
 
 ```