Update documentation for object_kind field in Webhook push and tag push Webhooks

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 7.9.0 (unreleased)
+  - Update documentation for object_kind field in Webhook push and tag push Webhooks (Stan Hu)
   - Fix broken email images (Hannes Rosenögger)
   - Fix mass SQL statements on initial push (Hannes Rosenögger)
   - Add tag push notifications and normalize HipChat and Slack messages to be consistent (Stan Hu)

doc/web_hooks/web_hooks.md

@@ -16,6 +16,7 @@ Triggered when you push to the repository except when pushing tags.
 
 ```json
 {
+  "object_kind": "push",
   "before": "95790bf891e76fee5e1747ab589903a6a1f80f22",
   "after": "da1560886d4f094c3e6c9ef40349f7d38b5d27d7",
   "ref": "refs/heads/master",
@@ -66,6 +67,7 @@ Triggered when you create (or delete) tags to the repository.
 
 ```json
 {
+  "object_kind": "tag_push",
   "ref": "refs/tags/v1.0.0",
   "before": "0000000000000000000000000000000000000000",
   "after": "82b3d5ae55f7080f1e6022629cdb57bfae7cccc7",