How to add a deprecation note about Symfony features

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In order to generate and log a warning message about a deprecated Symfony feature, use the trigger_error() function as follows:

trigger_error('The enctype method of the form helper was removed. You should use the new method start instead.', E_USER_DEPRECATED);

Please, be as explicit as possible in the deprecation message and always explain the alternative way to use that feature, property, method or class.

@CalinBolea noticed that we also have a part about deprecations in the contribution docs: http://symfony.com/doc/current/contributing/code/conventions.html#deprecations

Also the conventions about patches: http://symfony.com/doc/current/contributing/code/patches.html