Outdated OPACK docs

[rec0de]

Hi!

The OPACK protocol description (which was super helpful to us when testing the opack parser for bytewitch, btw) does not seem to match your source code in some details:

The doc specifies length field sizes for strings or bytes as 1, 2, 3, or 4 bytes (linear progression) when the actual behaviour we observe seems to use 1, 2, 4, or 8 bytes (exponential progression).

The OPACK tests already reflect this, so I assume it's just a matter of the documentation not being entirely up to date. https://github.com/postlund/pyatv/blob/c791f4a49b30bae2e7e6017b947224d0946c4d8d/tests/support/test_opack.py#L87

Either way, thank you for the detailed protocol docs ✨