Polish recent changes to webhook-triggers page

The following discussion from !197850 (merged) should be addressed:

• @aqualls started a discussion:

  @patrickbajao I'm approving. I could make some small tweaks to the docs here, but the entire page needs a review, and this merge request has nearly 70 comments on it.

  Turn this comment into a follow-up issue and I'll do a light edit on doc/user/project/integrations/webhook_events.md after your changes merge.