[bug]: better document mutually exclusive fields in MintAsset

[ZZiigguurraatt]

With the MintAsset (https://lightning.engineering/api-docs/api/taproot-assets/mint/mint-asset/) RPC, we have a number of fields in the MintAsset message which are not clearly documented.

• group_key and group_anchor I think may be mutually exclusive, but I'm not sure.
• new_grouped_asset and grouped_asset appear to be mutually exclusive, but that's not stated.

Please update the descriptions of these fields to make it more clear so that the user does not have to think so hard and do a lot of trial and error to figure it out.

[ZZiigguurraatt]

The answers are provided in #1517 (comment) and this issue can be merged with #1519