minor #7773 Add helpful remarks on custom DataCollector (Xosofox)

This PR was submitted for the master branch but it was merged into the 2.7 branch instead (closes #7773).

Discussion
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Add helpful remarks on custom DataCollector

When creating my own data colletor, I was not clear on how to exactly collect my data (unrelated to request/response)

Here are two remarks that would have helped me a lot by the time

* collect is only called once to "pick-up, not "gather"
* how/where to store and access data that is not related to request/response

Commits
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29522b3 Add helpful remarks on custom DataCollector

Author: xabbuh

profiler/data_collector.rst

@@ -30,6 +30,11 @@ The
 :method:`Symfony\\Component\\HttpKernel\\DataCollector\\DataCollectorInterface::collect`
 method is responsible for storing the collected data in local properties.
 
+.. caution::
+
+    The :method:`Symfony\\Component\\HttpKernel\\DataCollector\\DataCollectorInterface::collect` method is only called once.
+    It is not used to "gather" data but is there to "pick up" the data that has been stored by your service. 
+
 Most of the time, it is convenient to extend
 :class:`Symfony\\Component\\HttpKernel\\DataCollector\\DataCollector` and
 populate the ``$this->data`` property (it takes care of serializing the
@@ -71,6 +76,11 @@ collects the method and accepted content types from the request::
 
 The getters are added to give the template access to the collected information.
 
+.. caution::
+
+    If the data that is not directly related to the request or response, you need to make the data accessible to your DataCollector.
+    This can be achieved by injecting the service that holds the information you intend to profile into your DataCollector.
+
 .. caution::
 
     As the profiler serializes data collector instances, you should not