Q&A and Visual UGC responses now describe their fields

Retrieve all questions, Retrieve images by album, Retrieve product images, Retrieve custom albums and the other Q&A and Visual UGC endpoints now describe their response fields. Album image URLs are protocol-relative (they start with //), so add https: before using them. Retrieve all questions no longer lists page and per_page in its response, because the API doesn't return them. The Q&A bottom line endpoint is public: it now documents its 404 for an unknown product instead of a 401 it never returns.