Renamed empty_value to placeholder

| Q   | A
| --- | ---
| Doc fix? | no
| New docs? | yes (symfony/symfony#12003)
| Applies to | 2.6+
| Fixed tickets | #4261
Well, I think for the moment we should at least link to the reference of the `placeholder` option. But maybe later on someone writes a nice article about the `placeholder` and what else you can do with that (no matter how likely that is, there might be other reasons to change this later).

However, moving this to it's own file is not that important. But please add a link to the option's reference section.