Some of the constants that need documentation are below. Those that are available for the user to edit should also be added to the wiki and possibly the README.

For users:
MAVLINK_USE_CONVENIENCE_FUNCTIONS
MAVLINK_EXTERNAL_RX_BUFFER
MAVLINK_CRC_EXTRA
MAVLINK_ENDIAN
Various constants in protocol.h, mavlink.h

For developers:
MAVLINK_NEED_BYTE_SWAP
MAVLINK_ALIGNED_FIELDS
Various constants in protocol.h, mavlink.h
MAVLINK_MAX_EXTENDED_PAYLOAD_LEN - HW Note, this no longer exists