minor #5637 Add QueryBuilder vs DQL section (bocharsky-bw)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #5637).

Discussion
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Add QueryBuilder vs DQL section

Describe difference between QueryBuilder and DQL usage.

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes
| Applies to    | all
| Fixed tickets | none

Commits
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cfc5873 Add explanation when to use QueryBuilder instead of DQL

book/doctrine.rst

@@ -766,8 +766,10 @@ covered later), group, etc. For more information, see the official
 Querying for Objects Using Doctrine's Query Builder
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
-Instead of writing a DQL string, you can alternatively use a helpful object called
-the ``QueryBuilder`` to build that string for you::
+Instead of writing a DQL string, you can use a helpful object called the
+``QueryBuilder`` to build that string for you. This is useful when the actual query
+depends on dynamic conditions, as your code soon becomes hard to read with
+DQL as you start to concatenate strings::
 
     $repository = $this->getDoctrine()
         ->getRepository('AppBundle:Product');