minor #10550 [standards] Document "phpdoc_types_order" (phansys)

This PR was merged into the 2.8 branch.

Discussion
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[standards] Document "phpdoc_types_order"

See symfony/symfony#28780.

Commits
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7587534 [standards] Document "phpdoc_types_order"

Author: javiereguiluz

contributing/code/standards.rst

@@ -194,7 +194,10 @@ Structure
 
 * Do not use spaces around ``[`` offset accessor and before ``]`` offset accessor;
 
-* Add a ``use`` statement for every class that is not part of the global namespace.
+* Add a ``use`` statement for every class that is not part of the global namespace;
+
+* When PHPDoc blocks like `@param` or `@return` include ``null`` and declare multiple
+  types, place ``null`` always on the last position.
 
 Naming Conventions
 ~~~~~~~~~~~~~~~~~~