Document expectRevert() (#128)

Fixes foundry-rs/book#110

src/reference/cheatcodes.md

@@ -56,6 +56,7 @@ interface CheatCodes {
     function etch(address who, bytes calldata code) external;
     // Sets an address' code
 
+    function expectRevert() external;
     function expectRevert(bytes calldata) external;
     function expectRevert(bytes4) external;
     // Expects an error on next call
@@ -376,6 +377,24 @@ An alternative version of [`expectRevert`](#expectrevert) that only takes an err
 cheats.expectRevert(MyContract.CustomError.selector)
 ```
 
+##### Alternative Signature without error message
+
+```solidity
+function expectRevert() external;
+```
+
+If you need to test that a function reverts _without_ a message, you can do so with `expectRevert()`.
+
+###### Example
+
+```solidity
+function testExpectRevertNoReason() public {
+    Reverter reverter = new Reverter();
+    cheats.expectRevert();
+    reverter.revertWithoutReason();
+}
+```
+
 <br>
 
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