Instant Search: improve the "filter options" sidebar header

In the Instant Search sidebar, we always show the header 'Filter options' at the top. It is shown regardless of what is included in the sidebar.

I'm not sure it adds much visually or helps explain what the filters do. The phrase is also a bit odd. Some potential solutions:
- Rename it to 'Filters' and make sure it's only shown when filters are present
- Hide it in the modal and make the heading screen reader only

Start by locating the Instant Search sidebar and its modal presentation, then inspect where the “Filter options” heading is rendered. Decide which proposed behavior is supported by the surrounding UI and accessibility needs; done means the heading and filter presence behave consistently in both contexts.

Written by the indexing model from the issue text.