Add missing docstrings

contracts/MsgValueSimulator.sol

@@ -32,6 +32,10 @@ contract MsgValueSimulator is ISystemContract {
         to = address(uint160(addressAsUint));
     }
 
+    /// @notice The fallback function that is the main entry point for the MsgValueSimulator.
+    /// @dev The contract accepts value, the callee and whether the call should a system one via its ABI params.
+    /// @param _data The calldata to be passed to the callee.
+    /// @return The return data from the callee.
     fallback(bytes calldata _data) external onlySystemCall returns (bytes memory) {
         (uint256 value, bool isSystemCall, address to) = _getAbiParams();
 

contracts/NonceHolder.sol

@@ -147,6 +147,10 @@ contract NonceHolder is INonceHolder, ISystemContract {
         (prevDeploymentNonce, ) = _splitRawNonce(oldRawNonce);
     }
 
+    /// @notice A method that checks whether the nonce has been used before.
+    /// @param _address The address the nonce of which is being checked.
+    /// @param _nonce The nonce value which is check.
+    /// @return `true` if the nonce has been used, `false` otherwise.
     function isNonceUsed(address _address, uint256 _nonce) public view returns (bool) {
         uint256 addressAsKey = uint256(uint160(_address));
         return (_nonce < getMinNonce(_address) || nonceValues[addressAsKey][_nonce] > 0);