* 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>