minor #5840 [Contributing] [Standards] Add note about trigger_error() and deprecation messages (phansys)

This PR was merged into the 2.3 branch.

Discussion
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[Contributing] [Standards] Add note about `trigger_error()` and deprecation messages

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets |

Add note and example on how to trigger proper deprecation messages.
Related to #5525.

Commits
-------

4c2f1fb [Contributing] [Standards] Add note about `trigger_error()` and deprecation messages

Author: weaverryan

contributing/code/standards.rst

@@ -50,6 +50,18 @@ example containing most features described below:
             $this->fooBar = $this->transformText($dummy);
         }
 
+        /**
+         * @return string
+         *
+         * @deprecated
+         */
+        public function someDeprecatedMethod()
+        {
+            @trigger_error(sprintf('The %s() method is deprecated since version 2.8 and will be removed in 3.0. Use Acme\Baz::someMethod() instead.', __METHOD__), E_USER_DEPRECATED);
+
+            return Baz::someMethod();
+        }
+
         /**
          * Transforms the input given as first argument.
          *
@@ -151,7 +163,11 @@ Structure
 * Use parentheses when instantiating classes regardless of the number of
   arguments the constructor has;
 
-* Exception message strings should be concatenated using :phpfunction:`sprintf`.
+* Exception and error message strings should be concatenated using :phpfunction:`sprintf`.
+
+* Calls to :phpfunction:`trigger_error` with type ``E_USER_DEPRECATED`` should be
+  switched to opt-in via ``@`` operator.
+  Read more at :ref:`contributing-code-conventions-deprecations`;
 
 Naming Conventions
 ------------------