Select all rows with same ID when 1 column meets criteria
There are a few ways that you can achieve this...

You can use a subquery to return the docNum of the rows that match the criteria from RDRO and then use an outer query to get the rest; like so:

SELECT r.docNum, r.itemCode
  FROM (SELECT DISTINCT docNum
          FROM rdro
         WHERE itemCode = 'DDS200') AS a
  JOIN rdro AS r
    ON r.docNum = a.docNum;


Or, as pointed out by Aaron, you can also use a WHERE EXISTS to achieve the same result:

SELECT r.docNum, r.itemCode
  FROM rdro as r
 WHERE EXISTS (
         SELECT * 
           FROM rdro 
          WHERE docNum = r.docNum 
            AND itemCode = 'DDS200');


Or you can use a WHERE...IN:

SELECT r.docNum, r.itemCode
  FROM rdro as r
 WHERE r.docNum IN (
         SELECT docNum
           FROM rdro 
          WHERE itemCode = 'DDS200');


Or you could even do it straight in the JOIN:

SELECT r.docNum, r.itemCode
  FROM rdro as r
  JOIN rdro AS o
    ON r.docNum = o.docNum
   AND o.itemCode = 'DDS200';


Heck, for completeness lets even throw in a CTE version:

 WITH docs
 AS
 (SELECT docNum FROM RDRO WHERE itemCode = 'DDS200')
  SELECT r.docNum, r.itemCode
    FROM RDRO AS r
    JOIN docs AS d
      ON r.docNum = d.docNum;


Try each one and review the execution plans that are generated for them before choosing which one to go for. Normally if two query plans are the same I choose the query that is easier to maintain. 

Example of execution and results can be found on sqlfiddle.