docs: set a limit on credential line length

We recently introduced a way for credential helpers to add arbitrary state as part of the protocol. Set some limits on line length to avoid helpers passing extremely large amounts of data. While Git doesn't have a fixed parsing length, there are other tools which support this protocol and it's kind to allow them to use a reasonable fixed-size buffer for parsing. In addition, we would like to be moderate in our memory usage and imposing reasonable limits is helpful for that purpose. In the event a credential helper is incapable of storing its serialized state in 64 KiB, it can feel free to serialize it on disk and store a reference instead. Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>