Purge API comment (#23451) (#23452)

Backport #23451 by @jolheiser

This PR just adds the `purge` query parameter to the swagger docs for
admin user delete.

I considered using the same verbiage we have in the UI, but that seemed
more verbose than descriptions we use elsewhere in swagger. I'm fine if
that's preferred, though, just let me know. 🙂

Signed-off-by: jolheiser <john.olheiser@gmail.com>
Co-authored-by: John Olheiser <john.olheiser@gmail.com>

routers/api/v1/admin/user.go

@@ -305,6 +305,10 @@ func DeleteUser(ctx *context.APIContext) {
 	//   description: username of user to delete
 	//   type: string
 	//   required: true
+	// - name: purge
+	//   in: query
+	//   description: purge the user from the system completely
+	//   type: boolean
 	// responses:
 	//   "204":
 	//     "$ref": "#/responses/empty"