Document possible action types for the user activity feed API (#31196)

Resolves #31131.

It uses the the go-swagger `enum` property to document the activity
action types.

modules/structs/activity.go

@@ -8,6 +8,9 @@ import "time"
 type Activity struct {
 	ID     int64 `json:"id"`
 	UserID int64 `json:"user_id"` // Receiver user
+	// the type of action
+	//
+	// enum: create_repo,rename_repo,star_repo,watch_repo,commit_repo,create_issue,create_pull_request,transfer_repo,push_tag,comment_issue,merge_pull_request,close_issue,reopen_issue,close_pull_request,reopen_pull_request,delete_tag,delete_branch,mirror_sync_push,mirror_sync_create,mirror_sync_delete,approve_pull_request,reject_pull_request,comment_pull,publish_release,pull_review_dismissed,pull_request_ready_for_review,auto_merge_pull_request
 	OpType    string      `json:"op_type"`
 	ActUserID int64       `json:"act_user_id"`
 	ActUser   *User       `json:"act_user"`

templates/swagger/v1_json.tmpl

@@ -18178,7 +18178,37 @@
           "x-go-name": "IsPrivate"
         },
         "op_type": {
+          "description": "the type of action",
           "type": "string",
+          "enum": [
+            "create_repo",
+            "rename_repo",
+            "star_repo",
+            "watch_repo",
+            "commit_repo",
+            "create_issue",
+            "create_pull_request",
+            "transfer_repo",
+            "push_tag",
+            "comment_issue",
+            "merge_pull_request",
+            "close_issue",
+            "reopen_issue",
+            "close_pull_request",
+            "reopen_pull_request",
+            "delete_tag",
+            "delete_branch",
+            "mirror_sync_push",
+            "mirror_sync_create",
+            "mirror_sync_delete",
+            "approve_pull_request",
+            "reject_pull_request",
+            "comment_pull",
+            "publish_release",
+            "pull_review_dismissed",
+            "pull_request_ready_for_review",
+            "auto_merge_pull_request"
+          ],
           "x-go-name": "OpType"
         },
         "ref_name": {