XCM Doc Restructure: Covering pallet-xcm

[CrackTheCode016]

As part of issue #4541, this PR covers and documents pallet-xcm:

TODO

• What pallet-xcm does
• How pallet-xcm handles fees
• What extrinsics are important to note in pallet-xcm (and what they do)
• How pallet-xcm defines interfaces for XcmConfig
• Transfer Reserve vs Teleport difference

[joepetrowski]

This is not really clear

[CrackTheCode016]

• Transferring or reserving an asset implies that equivalent assets (i.e, native currency) are withdrawn from the ownership of the current origin and deposited into the sovereign account on the destination chain. Unlike teleporting an asset, it is not destroyed, rather a trusted entity is used (i.e., Statemint on Polkadot) to reserve the assets, wherein the sovereign account on the destination chain obtains ownership of them.

Is this clearer? Might be worth creating a diagram for to show the difference.

[KiChjang]

Is the term equivalent asset defined anywhere else other than here?

[CrackTheCode016]

Under the xcm-format repo, it's used a few times: https://github.com/paritytech/xcm-format#transferreserveasset

Is it worth noting that this asset is representative of the origin chain? i.e, DOT becoming known as "wrapped" DOT on the destination chain, or is that inaccurate?

[CrackTheCode016]

Just a sidenote - the goal of this portion is to cover some concepts that will be covered in more depth later in the wiki, (example: Trader is mentioned but not explained in depth quite yet). Generally, I think it's good to showcase pallet-xcm initially as an example of where to start when looking into the more technical aspects of understanding XCM.

[CrackTheCode016]

@KiChjang @joepetrowski Thank you both for the review!

If it looks good, I think it would be ready to polish slightly and merge. In subsequent XCM sections, more content and detail can be provided.