Is data transaction supports multisignature? Or will support?

If I look at
https://docs.wavesplatform.com/en/technical-details/data-transaction.html

proofs ? currently only signature is supported

After activation of Smart Accounts the Proofs of Data Transaction can be validated by Account script.
This is an outdated documentation, thank you for report!

It supports proofs also.
https://docs.wavesplatform.com/en/technical-details/data-structures.html#section-f6e7a2443d41af2a0ef8b4c4c33ba6b3