Add Documentation Regarding raw_packet_data, raw_value, and value

[medley56]

The definition of raw_value is especially fuzzy since it's not perfectly consistent between Data Encoding types.

Add a discussion of what we mean by

• raw packet data: full packet worth of bytes
• raw_value: as direct a representation as possible of the encoded value (int, float, bytes)
• value: calibrated, derived, fully parsed, enumerated lookup value. The value that the user probably cares about.