Document `object_attributes.oldrev` in the merge request webhook event

What does this MR do and why?

Adds to the documentation to describe when is the object_attributes.oldrev field available.

This field is not always present in the webhook body so it's good to know when it will appear.

Screenshots or screen recordings

None

How to set up and validate locally

This is just a documentation change so it should be enough to read the changed text.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

doc/user/project/integrations/webhook_events.md

@@ -824,10 +824,10 @@ The available values for `object_attributes.action` in the payload are:
 - `unapproval`
 - `merge`
 
-The field `object_attributes.oldrev` is only available when there are actual code changes, like
+The field `object_attributes.oldrev` is only available when there are actual code changes, like:
 
-- new code pushed
-- [suggestion](../merge_requests/reviews/suggestions.md) applied
+- New code is pushed.
+- A [suggestion](../merge_requests/reviews/suggestions.md) is applied.
 
 Request header: