feature #4047 Documented info method (WouterJ)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #4047).

Discussion
----------

Documented info method

| Q   | A
| --- | ---
| Doc fix? | no
| New docs? | yes (symfony/symfony#11453)
| Applies to | 2.6+
| Fixed tickets | -

Commits
-------

37d5361 Documented info method

components/config/definition.rst

@@ -287,6 +287,19 @@ has a certain value:
         ->end()
     ;
 
+Documenting the Option
+----------------------
+
+All options can be documented using the
+:method:`Symfony\\Component\\Config\\Definition\\Builder\\NodeDefinition::info`
+method.
+
+The info will be printed as a comment when dumping the configuration tree.
+
+.. versionadded:: 2.6
+    Since Symfony 2.6, the info will also be added to the exception message
+    when an invalid type is given.
+
 Optional Sections
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