Adding configuration example for retain modes (#3589)

* Adding configuration example for retain modes

Reading the documentation on its own didn't help me but when I found https://github.com/blakeblackshear/frigate/discussions/2447 I was able to understand how to add this to my configuration. I've added the example given in that discussion to help future readers of the page.

* Update record.md

Added suggested changes and have also added wording beneath the example mentioning the configuration can be added on a per camera basis.
Have also built on the example to add object specific retentions timings - Not sure if it would be preferred to have it all within one example to not complicate things?

Let me know your thoughts

* Update record.md

Created Object Specific Retention header

* Typo

Co-authored-by: Blake Blackshear <blake@frigate.video>

docs/docs/configuration/record.md

@@ -42,3 +42,35 @@ The same options are available with events. Let's consider a scenario where you
 - With the `all` option all segments for the duration of the event would be saved for the event. This event would have 4 hours of footage.
 - With the `motion` option all segments for the duration of the event with motion would be saved. This means any segment where a car drove by in the street, person walked by, lighting changed, etc. would be saved. 
 - With the `active_objects` it would only keep segments where the object was active. In this case the only segments that would be saved would be the ones where the car was driving up, you going inside, you coming outside, and the car driving away. Essentially reducing the 4 hours to a minute or two of event footage. 
+
+A configuration example of the above retain modes where all `motion` segments are stored for 7 days and `active objects` are stored for 14 days would be as follows:
+```yaml
+record:
+  enabled: True
+  retain:
+    days: 7
+    mode: motion
+  events:
+    retain:
+      default: 14
+      mode: active_objects
+```
+The above configuration example can be added globally or on a per camera basis.
+
+### Object Specific Retention
+
+You can also set specific retention length for an object type. The below configuration example builds on from above but also specifies that recordings of dogs only need to be kept for 2 days and recordings of cars should be kept for 7 days.
+```yaml
+record:
+  enabled: True
+  retain:
+    days: 7
+    mode: motion
+  events:
+    retain:
+      default: 14
+      mode: active_objects
+      objects:
+        dog: 2
+        car: 7
+```