minor #5446 [Contributing] [Standards] Added note about phpdoc_separation (phansys)

This PR was submitted for the 2.8 branch but it was merged into the 2.3 branch instead (closes #5446).

Discussion
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[Contributing] [Standards] Added note about phpdoc_separation

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

Commits
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fd7c335 [Contributing] [Standards] Added note about phpdoc_separation.

Author: xabbuh

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.