docs: fix coset index little-endian and correct panic conditions

[radik878]

• Switch documentation from big-endian to little-endian (LSB-first) for query_position_to_coset and query_position_to_coset_elements. Implementation uses pow_le and tests use to_bits_le, so docs must reflect little-endian to avoid misuse.
• Correct “Panics” sections: functions require at least (self.dim - coset_dim) bits; extra higher bits are ignored. Previous docs incorrectly referenced equality to coset_dim/self.dim.
• Clarify elements returns up to offset*g^{coset_size-1} (off-by-one in doc) and reword coset step comment for precision.