Document all fields for all messages

[chapulina]

All fields for all messages should have documentation that explains, where appropriate:

• Units
• Reference frame

• BCRecord
• PBCommandResponse
• PBRecord
• PBWatch
• PCRecord
• SCRecord
• TFRecord
• XBRecord (Use standard messages for AHRS #7)

[mabelzhang]

Update from group discussion:

• Possibly add messages that match buoy, even if those values don't appreciably change.
• Create messages for heave cone (TFRecord) and battery (BCRecord) in buoy_interfaces/msg

Update: This comment moved to #121