minor #3802 Add a note about which types can be used in Symfony (fabpot)

This PR was merged into the 2.3 branch.

Discussion
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Add a note about which types can be used in Symfony

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | all
| Fixed tickets | n/a

see symfony/symfony#10701

Commits
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bf9d509 added a note about types we support

contributing/code/standards.rst

@@ -133,6 +133,10 @@ Naming Conventions
 
 * Use alphanumeric characters and underscores for file names;
 
+* For type-hinting in PHPDocs and casting, use ``bool`` (instead of ``boolean``
+  or ``Boolean``), ``int`` (instead of ``integer``), ``float`` (instead of
+  ``double`` or ``real``);
+
 * Don't forget to look at the more verbose :doc:`conventions` document for
   more subjective naming considerations.