Add link to conventions#deprecations.

Author: phansys

contributing/code/standards.rst

@@ -166,7 +166,7 @@ Structure
 * 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;
+  switched to opt-in via `@` operator (see :doc:`conventions#deprecations` for more details);
 
 Naming Conventions
 ------------------