Added docs for ETH staking stake

Author: nkuba

solidity/contracts/ETHStaking.sol

@@ -20,7 +20,6 @@ import "@keep-network/keep-core/contracts/KeepRegistry.sol";
 
 import "openzeppelin-solidity/contracts/math/SafeMath.sol";
 
-
 /// @title ETH Staking
 /// @notice A staking contract for ETH staking. An owner of the ETH can delegate
 /// ETH as a stake to an operator. The value of ETH the owner is willing to stake
@@ -38,11 +37,18 @@ contract ETHStaking is Authorizations, StakeDelegatable {
         address indexed authorizer
     );
 
+    /// @notice Registers stake details. The function is used to register
+    /// addresses of operator, beneficiary and authorizer for a stake from the
+    /// caller.
+    /// @param operator Address of the operator.
+    /// @param beneficiary Address of the beneficiary.
+    /// @param authorizer Address of the authorizer.
     function stake(
         address operator,
         address payable beneficiary,
         address authorizer
     ) external payable {
+        // TODO: Do we need to verify the caller in any way?
         address _from = msg.sender;
 
         require(