Wrong/unclear explanation in IndexedMap docs

[albertchon]

The docs for IndexedMap state

cw-plus/packages/storage-plus/README.md

Lines 488 to 489 in 119032e

	Given that `token_id` is a string value, we specify `String` as the last argument of the `MultiIndex` definition.
	That way, the deserialization of the primary key will be done to the right type (an owned string).

However in the latest cw-nfts code (using cw-storage-plus v0.13.2), this is no longer the case
https://github.com/CosmWasm/cw-nfts/blob/584e01526048334b87a85a6c69f54109727efb5a/contracts/cw721-base/src/state.rs#L121-L126

It seems this change arose from @maurolacy's removing of the PK default from MultiIndex, so that "the PK type must match the encompassing IndexedMap primary key type, or its owned variant".

[maurolacy]

cw-nfts code should be modified to conform to this new format / convention, yes.

Please note this is a convention, and no compilation error will be thrown if you don't follow it. You will most likely hit deserialisation errors, and / or range / iteration misses, particularly if you indicate deserialising the pk to an incompatible type through this.