Document that endianness affects bit field extraction order

[keichi]

• Add a note to the endianness() API documentation that the setting also affects bit field extraction order
• In big-endian mode, bit fields are extracted from MSB to LSB; in little-endian mode, from LSB to MSB
• Mention the workaround for controlling byte order and bit order independently (using .nest() with a separate Parser)

Closes #289