log_processor/data_sources/victorialogs: update parameters description

Update parameters description in sync with crowdsecurity/crowdsec#3924

Author: zekker6

crowdsec-docs/docs/log_processor/data_sources/victorialogs.md

@@ -14,6 +14,7 @@ mode: tail
 log_level: info
 url: http://localhost:9428/
 limit: 1000
+since: 1m
 query: |
   app:nginx
 auth:
@@ -23,10 +24,6 @@ labels:
  type: nginx
 ```
 
-:::info
-The reader will always start at "now" for `tail` mode.
-:::
-
 Look at the `configuration parameters` to view all supported options.
 
 ## Parameters
@@ -96,6 +93,10 @@ The maximum duration VictoriaLogs is allowed to be unavailable (once startup is
 
 Default to `30 seconds`.
 
+### `since`
+
+Defines an offset duration to apply to the "start" time of VictoriaLogs query.
+This is useful in order to avoid missing logs when restarting crowdsec. Note that it is possible to have logs processed multiple times when using this parameter.
 
 ## DSN and command-line