In Doc, suggest to remind user to include the port if not default for Github OAuth Application

In #29360 (closed), I met trouble with Github OAuth which is caused by the mismatch of callback URL and actual redirect URI. The reason is that Gitlab setup is on non-default port but the Github OAuth Application does not include that port. Suggest to remind users to have the Gitlab port included if running it on non-default port on Github integration doc for ce and for ee.