Fix _validateSignatures NatSpec Documentation (#183)

This PR fixes the NatSpec documentation for the `_validateSignatures`
internal function. This fixes _I6_ from the Ackee audit v2.0.

4337/contracts/Safe4337Module.sol

@@ -167,7 +167,11 @@ contract Safe4337Module is IAccount, HandlerContext, CompatibilityFallbackHandle
     }
 
     /**
-     * @dev Validates that the user operation is correctly signed. Reverts if signatures are invalid.
+     * @dev Validates that the user operation is correctly signed and returns an ERC-4337 packed validation data
+     * of `validAfter || validUntil || authorizer`:
+     *  - `authorizer`: 20-byte address, 0 for valid signature or 1 to mark signature failure (this module does not make use of signature aggregators).
+     *  - `validUntil`: 6-byte timestamp value, or zero for "infinite". The user operation is valid only up to this time.
+     *  - `validAfter`: 6-byte timestamp. The user operation is valid only after this time.
      * @param userOp User operation struct.
      * @return validationData An integer indicating the result of the validation.
      */