[Translation] added logging capability.

| Q             | A
| ------------- | ---
| Doc fix?	 | no
| New docs? | yes(symfony/symfony/pull/10887)
| Applies to | 2.6+
|Fixed tickets |	-
	
This is our one spot to give a few more details. Please update the wording as you see fit!

>
When ``true``, a log entry is made whenever the translator cannot find a translation
for a given key. The logs are made to the `translation` channel and at the `debug`
for level for keys where there is a translation in the fallback locale and the `warning`
level if there is no translation to use at all.