* Adds / Updates documentation of ERC721 contract

* Improve ERC721Burnable documentation

* Fix typo

* Revert changes on ERC721 private functions

* Add documentation to the ERC721 contract's constructor

* Add IERC721Receiver & ERC721Holder documentations

* Add references to IERC721 functions

* Add references to IERC721Metadata/Receiver

* PR fixes

* Fixes to ERC721 documentation

* Add missing fixes

Co-authored-by: Nicolás Venturo <nicolas.venturo@gmail.com>