HTTP API documentation +fix when updating a playlist (#12612)

* get id from path param when updating a playlist

* add playlist http api documentation

* remove required condition for the id in update cmd

pkg/api/playlist.go

@@ -160,6 +160,7 @@ func CreatePlaylist(c *m.ReqContext, cmd m.CreatePlaylistCommand) Response {
 
 func UpdatePlaylist(c *m.ReqContext, cmd m.UpdatePlaylistCommand) Response {
 	cmd.OrgId = c.OrgId
+	cmd.Id = c.ParamsInt64(":id")
 
 	if err := bus.Dispatch(&cmd); err != nil {
 		return Error(500, "Failed to save playlist", err)