docs: Add prevrandao cheatcode (#889)

* Update cheatcodes README.md interface

* add `prevrandao.md` cheatcode description

* chore: add more context on docs

src/cheatcodes/README.md

@@ -60,7 +60,12 @@ interface CheatCodes {
     function fee(uint256) external;
 
     // Set block.difficulty
+    // Does not work from the Paris hard fork and onwards, and will revert instead.
     function difficulty(uint256) external;
+    
+    // Set block.prevrandao
+    // Does not work before the Paris hard fork, and will revert instead.
+    function prevrandao(bytes32) external;
 
     // Set block.chainid
     function chainId(uint256) external;
@@ -250,6 +255,9 @@ interface CheatCodes {
 
     // Label an address in test traces
     function label(address addr, string calldata label) external;
+    
+    // Retrieve the label of an address
+    function getLabel(address addr) external returns (string memory);
 
     // When fuzzing, generate new inputs if conditional not met
     function assume(bool) external;

src/cheatcodes/difficulty.md

@@ -10,6 +10,8 @@ function difficulty(uint256) external;
 
 Sets `block.difficulty`.
 
+If used with a post-merge EVM version (Paris and onwards), it will revert. In that case, use [`vm.prevrandao`](./prevrandao.md) instead.
+
 ### Examples
 
 ```solidity

src/cheatcodes/prevrandao.md

@@ -0,0 +1,20 @@
+## `prevrandao`
+
+### Signature
+
+```solidity
+function prevrandao(bytes32) external;
+```
+
+### Description
+
+Sets `block.prevrandao`.
+
+If used with an EVM version previous to the Paris hard fork, it will revert. In that case, use [`vm.difficulty`](./difficulty.md) instead.
+
+### Examples
+
+```solidity
+vm.prevrandao(bytes32(uint256(42)));
+emit log_uint(block.prevrandao); // 42
+```