document kafka partitionLimitation parameter (#981)

Signed-off-by: Tobias Krause <tobias.krause@otto.de>
Signed-off-by: Tobias Krause <tobiaskrause@users.noreply.github.com>
Co-authored-by: Tom Kerkhove <kerkhove.tom@gmail.com>

content/docs/2.8/scalers/apache-kafka.md

@@ -32,6 +32,7 @@ triggers:
     allowIdleConsumers: false
     scaleToZeroOnInvalidOffset: false
     version: 1.0.0
+    partitionLimitation: '1,2,10-20,31'
 ```
 
 **Parameter list:**
@@ -48,6 +49,7 @@ partitions on a topic, allowing for idle consumers. (Default: `false`, Optional)
 If 'false' (the default), the scaler will keep a single consumer for that partition. Otherwise ('true'), the consumers for that
 partition will be scaled to zero. See the [discussion](https://github.com/kedacore/keda/issues/2612) about this parameter.
 - `version` - Version of your Kafka brokers. See [samara](https://github.com/Shopify/sarama) version (Default: `1.0.0`, Optional)
+- `partitionLimitation` - Comma separated list of partition ids to scope the scaling on. Allowed patterns are "x,y" and/or ranges "x-y". If set, the calculation of the lag will only take these ids into account.  (Default: All partitions, Optional) 
 
 > **Note:**
 >