Move date suggestion text out of placeholder

Multiple changes here:
- The date filters are now way more flexible than they were, so I've changed the suggestions to show two accepted date formats as examples.
- Use "for example" instead of eg because in previous filters, users seemed to treat eg as ie, meaning they would only use the search box with the suggested format types. Since we can fit "for example" in this placeholder text we should use it instead.
- To avoid replication of the same text, and because the date fields are already visually linked by the grey box, have one helptext at the bottom of the box, rather than one for each input.
Merge pull request #124 from alphagov/move-suggested-dates-on-date-fields

Move date suggestion text out of placeholder