Update ReentrancyGuard documentation (#3715)

(cherry picked from commit 06e678d6ef966d9c55d512b736622682bcfdf7a7)

contracts/security/ReentrancyGuard.sol

@@ -54,7 +54,7 @@ abstract contract ReentrancyGuard {
     }
 
     function _nonReentrantBefore() private {
-        // On the first call to nonReentrant, _notEntered will be true
+        // On the first call to nonReentrant, _status will be _NOT_ENTERED
         require(_status != _ENTERED, "ReentrancyGuard: reentrant call");
 
         // Any calls to nonReentrant after this point will fail