[Contributing] [Standards] Added note about phpdoc_separation.

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | 2.8+
| Fixed tickets |

Author: phansys

contributing/code/standards.rst

@@ -200,6 +200,10 @@ Documentation
 
 * Add PHPDoc blocks for all classes, methods, and functions;
 
+* Group annotations together so that annotations of the same type immediately
+  follow each other, and annotations of a different type are separated by a
+  single blank line;
+
 * Omit the ``@return`` tag if the method does not return anything;
 
 * The ``@package`` and ``@subpackage`` annotations are not used.