docs: clarify path_length hash-size encoding

[robekl]

Summary

• document that \ encodes hop count in bits 0-5 and path hash size in bits 6-7
• clarify that path byte length is computed as hop_count * hash_size rather than taken directly from \
• add examples for 1-byte, 2-byte, and 3-byte path hash encodings

Testing

• not run (documentation-only change)