[patch] curl_setopt option table headings

I've been using this function reference for years, and was surprised to find that if I read closely, above each table for the various values to pass in, the type of the value is specified. I'd never read the page closely enough to realize that the options are grouped by type; I think it might be common for readers to skim like this. I think it'll be more obvious to the reader if the types are specified as a title of each table. This is my first foray into docbook, so comments on how to achieve this are appreciated! Cheers, Patrick