Clarify warning message

deps/rabbit/src/rabbit_disk_monitor.erl

@@ -236,7 +236,9 @@ get_disk_free(Dir, {win32, _}) ->
     % "c:/Users/username/AppData/Roaming/RabbitMQ/db/rabbit2@username-z01-mnesia"
     case win32_get_drive_letter(Dir) of
         error ->
-            rabbit_log:warning("Mnesia directory is not in the expected format (C:): '~p'", [Dir]),
+            rabbit_log:warning("Expected the mnesia directory absolute "
+                               "path to start with a drive letter like "
+                               "'C:'. The path is: '~p'", [Dir]),
             case win32_get_disk_free_dir(Dir) of
                 {ok, Free} ->
                     Free;