[RFC][WebProfilerBundle] Add simple placeholders into search form

This is an RFC mostly, it's not a real feature, it's more like little helper for the profiler form, I guess that would mostly useful for _newcomers_, but sometimes even _old dogs_ could find it useful sometimes =)

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | kinda
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

![form](https://cloud.githubusercontent.com/assets/67402/2655386/4637aa0c-bfe5-11e3-958e-84308217b52e.png)