Clarify the logger's MODE config option

docs/content/administration/logging-config.en-us.md

@@ -102,8 +102,11 @@ MODE = file, file-error
 
 ; by default, the "file" mode will record logs to %(log.ROOT_PATH)/gitea.log, so we don't need to set it
 ; [log.file]
+; by default, the MODE (actually it's the output writer of this logger) is taken from the section name, so we don't need to set it either
+; MODE = file
 
 [log.file-error]
+MODE = file
 LEVEL = Error
 FILE_NAME = file-error.log
 ```

modules/setting/log.go

@@ -165,7 +165,7 @@ func loadLogModeByName(rootCfg ConfigProvider, loggerName, modeName string) (wri
 		writerMode.WriterOption = writerOption
 	default:
 		if !log.HasEventWriter(writerType) {
-			return "", "", writerMode, fmt.Errorf("invalid log writer type (mode): %s", writerType)
+			return "", "", writerMode, fmt.Errorf("invalid log writer type (mode): %s, maybe it needs something like 'MODE=file' in [log.%s] section", writerType, modeName)
 		}
 	}