docs: urls::errc reference as namespace alias

fix boostorg#844

include/boost/url/error_types.hpp

@@ -196,31 +196,6 @@ error_category const& system_category();
 using boost::system::system_category;
 #endif
 
-/** The set of constants used for cross-platform error codes
-
-    @warning This alias is no longer supported and
-    should not be used in new code. Please use
-    `core::string_view` instead.
-
-    This alias is included for backwards
-    compatibility with earlier versions of the
-    library.
-
-    However, it will be removed in future releases,
-    and using it in new code is not recommended.
-
-    Please use the updated version instead to
-    ensure compatibility with future versions of
-    the library.
-
- */
-#ifdef BOOST_URL_DOCS
-enum errc
-{
-    __see_below__
-};
-#else
-
 /** The set of constants used for cross-platform error codes
 
     @warning This alias is no longer supported and
@@ -240,7 +215,6 @@ enum errc
 
  */
 namespace errc = boost::system::errc;
-#endif
 
 /** The type of result returned by library functions