Improve pallet-xcm-precompile docs (assets ordering)

[Agusrodri]

What does it do?

Adds an extra line of description to the functions present in pallet-xcm-precompile's interface.

Basically, if we send assets that are not sorted and we specify a fee index, it may happen that the incorrect asset is used to pay for fees. This is because internally, we need to perform some conversions with the Assets struct, which automatically sorts the assets being converted.

The purpose of the new extra line of docs is to inform (and prevent) users about this behavior.

[gonzamontiel]

I see this as a necessary patch but I would soon add a function to automatically sort the assets.
Also, the type of sort that the users need to use is implicit. While it might be obvious, I would take the opportunity to be more precise (and maybe add an example).

[stiiifff]

I would specify that the order matters, and as you mention, that the fee index relates to the asset array.

[Agusrodri]

As discussed internally, we will add some examples for the assets ordering, and will work on a future PR containing the helper function to automatically sort the assets.

[RomarQ]

Adding some context to this issue, the explanation below is copied from: paritytech/polkadot-sdk#2388 (comment)

MultiAssets object internally sorts the list of assets (I'm not sure where this requirement to be sorted comes from, maybe required for encoding/decoding?).

So my guess is you're doing something like:

let assets: MultiAssets = vec![TEER, ROC].into();    // <-- this works
let fee_idx = 1;                                     // ok

let assets: MultiAssets = vec![ROC, TEER].into();    // <-- still works, but `fee_idx` should still be `1` because of sorting :(
let fee_idx = 0; // <-- reading this looks like it's pointing to ROC, but because of the sorting done by `.into()`, TEER is actually idx 0 and ROC is idx 1

(local assets will always go first because of parents=0 vs ROC which has parents=1)

and I know... this is annoying and bad API design - we could change extrinsic to take actual fee asset instead of index, but would be a breaking change..

Also here is an example helper function I used in tests to get the right index for transfers.