At the time of writing (January 2022) there hasn't been a descriptor specified for Schnorr/Taproot multisig (using the new OP_CHECKSIGADD opcode specified in BIP 342) or using the key aggregation scheme MuSig.
(When I say “multisig” I mean k-of-n threshold signing, rather than a schnorr multisignature)
A clarification on terminology. In Bitcoin we generally use "multisig" to refer to n-of-n (n=n) and "threshold" to refer to k-of-n (k<n). MuSig(2) refers to a key aggregation scheme where only one key and one signature go onchain despite there being multiple keys involved offchain. Threshold equivalents of MuSig(2) are being worked on e.g. FROST but are not finalized or as mature as MuSig(2). Neither MuSig(2) or FROST have descriptors specified at the time or writing.
Hence if you want to use a multisig or threshold signature descriptor you would use an existing one e.g.
sortedmulti that doesn't support a Schnorr/Taproot multisig. A bit confusingly the
multi descriptor does support both multisignature and threshold signatures according to the above definitions but not for Schnorr/Taproot.
April 2023 Update
There are now
sortedmulti_a descriptors supported in the Bitcoin Core wallet for k-of-n policies using the CHECKSIGADD opcode inside the
tr() Taproot descriptor. A MuSig(2) descriptor has been discussed and there is an open pull request in rust-miniscript implementing it but it hasn't yet been finalized.