Document secp256k1 wrapper function behaviors

[FreeOnlineUser]

While implementing BIP-352/374 Silent Payments for SeedSigner (PR #864), I found that the secp256k1 ctypes wrapper functions have no docstrings, which made it difficult to understand their behavior.

For example, ec_pubkey_negate():

from embit.util import secp256k1
help(secp256k1.ec_pubkey_negate)
# Output: wrapper(*args, **kwargs) - no documentation

Through trial and error, I discovered:

• ec_pubkey_negate() returns a new negated point (does not modify in-place)
• Input must be a parsed pubkey (from ec_pubkey_parse()), output is also parsed
• Need to call ec_pubkey_serialize() to get bytes back

Similar clarity would help for other functions like ec_pubkey_tweak_add(), ec_pubkey_combine(), etc.

Example docstring:

def ec_pubkey_negate(pubkey):
    """
    Negate a parsed public key (flip Y coordinate).
    
    Args:
        pubkey: Parsed pubkey from ec_pubkey_parse()
    
    Returns:
        New parsed pubkey (negated). Does NOT modify input.
        Call ec_pubkey_serialize() to get bytes.
    
    Example:
        parsed = secp256k1.ec_pubkey_parse(pubkey_bytes)
        negated = secp256k1.ec_pubkey_negate(parsed)
        result = secp256k1.ec_pubkey_serialize(negated)
    """

This would help developers implementing advanced Bitcoin protocols (Silent Payments, MuSig, etc.).