Remove BIP 174's claim that Combine is commutative

[shesek]

The BIP asserts that fA(fB(psbt)) == fB(fA(psbt)), however the explanatory text before this doesn't actually say this and even hints that the ordering does matter: "processing [..] A and then B in a sequence". It seems that the BIP text only supports the Combine(fA(psbt), fB(psbt)) == fB(fA(psbt)) part, and that fA(fB(psbt)) slipped in by accident?

In practice, Bitcoin Core's combinepsbt isn't commutative and gives precedence to latter PSBTs in the array. Here's a quick example demonstrating this:

PSBT_A="cHNidP8BADMCAAAAAa83fnb+Vw0gR7jZBeABQNh2dFx8F+MbmvHAM8N5+O07AAAAAAD/////AAAAAAAAAQUBUQA="
PSBT_B="cHNidP8BADMCAAAAAa83fnb+Vw0gR7jZBeABQNh2dFx8F+MbmvHAM8N5+O07AAAAAAD/////AAAAAAAAAQUBUgA="
$ bitcoin-cli decodepsbt $(bitcoin-cli combinepsbt [\"$PSBT_A\",\"$PSBT_B\"]) | jq -r .inputs[0].witness_script.asm
1
$ bitcoin-cli decodepsbt $(bitcoin-cli combinepsbt [\"$PSBT_B\",\"$PSBT_A\"]) | jq -r .inputs[0].witness_script.asm
2

And here's a related discussion about rust-bitcoin's Psbt::combine(), which isn't commutative either but documented as "In accordance with BIP 174 this function is commutative": rust-bitcoin/rust-bitcoin#5486

[murchandamus]

cc: @achow101