I'm not sure if there is a documentation but you can add it to your website by clicking on Add Includelet and selecting Custom Site Search in the Layout tab for your chosen page. 

You're right, Custom Site Search is kinda new and hasn't made it into the documentation. The general idea is that it adds a search box to your site, and you can configure it to search only within certain content sections.

https://www.bromleywell.org.uk/ have used it on their homepage to create a search box that only returns results from their Advice and Services sections, and not from all their news and events etc.

There is however no way to make search only match against page titles. Internally the titles, keywords, description, and content are all indexed together and can't be separated out.

However, note that title, description, and keywords are given a greater weight in the scoring algorithm, so pages that have matches here will be returned higher in the search results.

Thanks

Joe