Improve documentation of Initializable getters (#3861)

Co-authored-by: Ernesto García <ernestognw@gmail.com>

contracts/proxy/utils/Initializable.sol

@@ -150,14 +150,14 @@ abstract contract Initializable {
     }
 
     /**
-     * @dev Internal function that returns the initialized version. Returns `_initialized`
+     * @dev Returns the highest version that has been initialized. See {reinitializer}.
      */
     function _getInitializedVersion() internal view returns (uint8) {
         return _initialized;
     }
 
     /**
-     * @dev Internal function that returns the initialized version. Returns `_initializing`
+     * @dev Returns `true` if the contract is currently initializing. See {onlyInitializing}.
      */
     function _isInitializing() internal view returns (bool) {
         return _initializing;