Restructure intro to ERC721 contract docs

contracts/token/ERC721/README.adoc

@@ -7,18 +7,27 @@ This set of interfaces, contracts, and utilities are all related to the https://
 
 TIP: For a walk through on how to create an ERC721 token read our xref:ROOT:erc721.adoc[ERC721 guide].
 
-The EIP consists of three interfaces, found here as {IERC721}, {IERC721Metadata}, and {IERC721Enumerable}. Only the first one is required in a contract to be ERC721 compliant. The core interface and the metadata extension are both implemented in {ERC721}. The enumerable extension is provided separately in {ERC721Enumerable}.
+The EIP specifies four interfaces:
 
-Additionally, {IERC721Receiver} can be used to prevent tokens from becoming forever locked in contracts. Imagine sending an in-game item to an exchange address that can't send it back!. When using <<IERC721-safeTransferFrom,`safeTransferFrom`>>, the token contract checks to see that the receiver is an {IERC721Receiver}, which implies that it knows how to handle {ERC721} tokens. If you're writing a contract that needs to receive {ERC721} tokens, you'll want to include this interface.
+* {IERC721}: Core functionality required in all compliant implementation.
+* {IERC721Metadata}: Optional extension that adds name, symbol, and token URI, almost always included.
+* {IERC721Enumerable}: Optional extension that allows enumerating the tokens on chain, often not included since it requires large gas overhead.
+* {IERC721Receiver}: An interface that must be implemented by contracts if they want to accept tokens through `safeTransferFrom`.
 
-Additionally there are multiple custom extensions, including:
+OpenZeppelin Contracts provides implementations of all four interfaces:
 
-* designation of addresses that can pause token transfers for all users ({ERC721Pausable}).
-* destruction of own tokens ({ERC721Burnable}).
-* support for voting and vote delegation ({ERC721Votes})
+* {ERC721}: The core and metadata extensions, with a base URI mechanism.
+* {ERC721Enumerable}: The enumerable extension.
+* {ERC721Holder}: A bare bones implementation of the receiver interface.
 
-NOTE: This core set of contracts is designed to be unopinionated, allowing developers to access the internal functions in ERC721 (such as <<ERC721-_mint-address-uint256-,`_mint`>>) and expose them as external functions in the way they prefer. On the other hand, xref:ROOT:erc721.adoc#Presets[ERC721 Presets] (such as {ERC721PresetMinterPauserAutoId}) are designed using opinionated patterns to provide developers with ready to use, deployable contracts.
+Additionally there are a few of other extensions:
+
+* {ERC721URIStorage}: A more flexible but more expensive way of storing metadata.
+* {ERC721Votes}: Support for voting and vote delegation.
+* {ERC721Pausable}: A primitive to pause contract operation.
+* {ERC721Burnable}: A way for token holders to burn their own tokens.
 
+NOTE: This core set of contracts is designed to be unopinionated, allowing developers to access the internal functions in ERC721 (such as <<ERC721-_mint-address-uint256-,`_mint`>>) and expose them as external functions in the way they prefer. On the other hand, xref:ROOT:erc721.adoc#Presets[ERC721 Presets] (such as {ERC721PresetMinterPauserAutoId}) are designed using opinionated patterns to provide developers with ready to use, deployable contracts.
 
 == Core