Match ABI tag order in namespace tests to abi_macros.hpp

NLOHMANN_JSON_ABI_TAGS concatenates the tags as _diag, _ldvcmp, _dp,
but the default and noversion ABI tests expected _diag, _dp, _ldvcmp.
The tests therefore failed whenever both JSON_DIAGNOSTIC_POSITIONS and
JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON were enabled, a combination
CI never exercises. Reorder the expectations to match the header.

Also document the _dp tag in the namespace feature page, which listed
only _diag and _ldvcmp.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/abi/config/default.cpp

@@ -24,14 +24,14 @@ TEST_CASE("default namespace")
         expected += "_diag";
 #endif
 
-#if JSON_DIAGNOSTIC_POSITIONS
-        expected += "_dp";
-#endif
-
 #if JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON
         expected += "_ldvcmp";
 #endif
 
+#if JSON_DIAGNOSTIC_POSITIONS
+        expected += "_dp";
+#endif
+
         expected += "_v" STRINGIZE(NLOHMANN_JSON_VERSION_MAJOR);
         expected += "_" STRINGIZE(NLOHMANN_JSON_VERSION_MINOR);
         expected += "_" STRINGIZE(NLOHMANN_JSON_VERSION_PATCH) "::basic_json";