[fix lint warnings: NFTs pallet] fix clippy::missing_docs_in_private_items warnings (#14610)

* add docs for impl_codec_bitflags

* add missing docs for type aliases

* add docs to transfer module

* add docs for settings module

* add docs to roles module

* add docs to metadata module

* add docs to migration module

* add missing docs to feature library

* methods not functions

* add docs to lock module

* add docs to attributes module

* add docs to create_delete_item module

* add docs for create_delete_collection module

* add docs to buy_sell module

* add missing doc for buy_sell module

* add docs to atomic_swap module

* add docs to atomic_swap module

* add docs for approvals module

* run cargo fmt

* Fix issues with multi-line comments

* Apply suggestions from code review

Co-authored-by: Jegor Sidorenko <[email protected]>

* update from review

* fmt

* update from review

* remove bitflag example

* ".git/.scripts/commands/fmt/fmt.sh"

* Apply suggestions from code review

Co-authored-by: Squirrel <[email protected]>

* add note about pallet features

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Co-authored-by: Jegor Sidorenko <[email protected]>
Co-authored-by: Jegor Sidorenko <[email protected]>
Co-authored-by: parity-processbot <>
Co-authored-by: Squirrel <[email protected]>