Docs: Add ERC4907A docs (#380)

* Add ERC4907 docs

* Add note on retaining user info upon transfer

* Nits

docs/erc4907a.md

@@ -0,0 +1,77 @@
+# ERC4907A
+
+[`erc721a/contracts/extensions/ERC4907A.sol`](https://github.com/chiru-labs/ERC721A/blob/main/contracts/extensions/ERC4907A.sol)
+
+[ERC4907](https://eips.ethereum.org/EIPS/eip-4907) compliant extension of ERC721A, which allows owners and authorized addresses to add a time-limited role with restricted permissions to ERC721 tokens.
+
+This implementation does **not** reset the user information upon token transfer. 
+The new owner must call [`setUser`](#setUser) to change or reset the user.
+
+To reset or alter the user information after each transfer, you will need to override either 
+[`_beforeTokenTransfers`](erc721a.md#_beforeTokenTransfers) or 
+[`_afterTokenTransfers`](erc721a.md#_afterTokenTransfers) in ERC721A.
+
+Inherits:
+
+- [ERC721A](erc721a.md)
+- [IERC4907A](interfaces.md#ierc4907a) 
+
+
+## Functions
+
+### setUser
+
+```solidity
+function setUser(uint256 tokenId, address user, uint64 expires) public virtual
+```
+
+Sets the `user` and `expires` for `tokenId`.
+
+The zero address indicates there is no user.
+
+Requirements:
+
+- The caller must own `tokenId` or be an approved operator.
+
+
+### userOf 
+
+```solidity
+function userOf(uint256 tokenId) public view virtual returns (address)
+```
+
+Returns the user address for `tokenId`.
+
+The zero address indicates that there is no user or if the user is expired.
+
+### userExpires
+
+```solidity
+function userExpires(uint256 tokenId) public view virtual returns (uint256)
+```
+
+Returns the user's expires of `tokenId`.
+
+### \_explicitUserOf
+
+```solidity
+function _explicitUserOf(uint256 tokenId) internal view virtual returns (address)
+```
+
+Returns the user address for `tokenId`, ignoring the expiry status.
+
+
+## Events
+
+### UpdateUser
+
+`IERC4907-UpdateUser`
+
+```solidity
+event UpdateUser(uint256 indexed tokenId, address indexed user, uint64 expires)
+```
+
+Emitted when the `user` of an NFT or the `expires` of the `user` is changed.
+
+The zero address for user indicates that there is no user address.
+

docs/erc721a.md

@@ -465,6 +465,58 @@ If set, the resulting URI for each token will be the concatenation of the `baseU
 Empty by default, it can be overridden in child contracts.
 
 
+### \_beforeTokenTransfers
+
+```solidity
+function _beforeTokenTransfers(
+    address from,
+    address to,
+    uint256 startTokenId,
+    uint256 quantity
+) internal virtual
+```
+
+Hook that is called before a set of serially-ordered token IDs are about to be transferred. This includes minting.
+
+Also called before burning one token.
+
+`startTokenId` - the first token ID to be transferred.  
+`quantity` - the amount to be transferred.
+
+Calling conditions:
+
+- When `from` and `to` are both non-zero, `from`'s `tokenId` will be transferred to `to`.
+- When `from` is zero, `tokenId` will be minted for `to`.
+- When `to` is zero, `tokenId` will be burned by `from`.
+- `from` and `to` are never both zero.
+
+
+### \_afterTokenTransfers
+
+```solidity
+function _afterTokenTransfers(
+    address from,
+    address to,
+    uint256 startTokenId,
+    uint256 quantity
+) internal virtual
+```
+
+Hook that is called after a set of serially-ordered token IDs are about to be transferred. This includes minting.
+
+Also called after burning one token.
+
+`startTokenId` - the first token ID to be transferred.  
+`quantity` - the amount to be transferred.
+
+Calling conditions:
+
+- When `from` and `to` are both non-zero, `from`'s `tokenId` will be transferred to `to`.
+- When `from` is zero, `tokenId` will be minted for `to`.
+- When `to` is zero, `tokenId` will be burned by `from`.
+- `from` and `to` are never both zero.
+
+
 ### \_toString
 
 ```solidity

docs/interfaces.md

@@ -43,3 +43,16 @@ Inherits:
 import 'erc721a/contracts/interfaces/IERC721AQueryable.sol';
 
 ```
+
+### IERC4907A
+
+[`erc721a/contracts/extensions/IERC4907A.sol`](https://github.com/chiru-labs/ERC721A/blob/main/contracts/extensions/IERC4907A.sol)
+
+Inherits:
+
+- [IERC721A](#ierc721a)
+
+```solidity
+import 'erc721a/contracts/interfaces/IERC4907A.sol';
+
+```

docs/sidebar.md

@@ -7,6 +7,7 @@
   - [ERC721A](erc721a.md)
   - [ERC721ABurnable](erc721a-burnable.md)
   - [ERC721AQueryable](erc721a-queryable.md)
+  - [ERC4907A](erc4907a.md)
   - [Interfaces](interfaces.md)
 - **Theme**
 <div id="theme">