Doc: update user documentation of webhook log

A follow-up issue of https://gitlab.com/gitlab-org/gitlab/-/issues/340027.

After the MR (gitlab-org/gitlab!88737 (merged)) is merged, we'll need to update user documentation to reflect the limit and stripping data. Link to webhook documentation: https://docs.gitlab.com/ee/user/project/integrations/webhooks.html#troubleshoot-webhooks.