PostgreSQL does not seem to be available, despite their ad copy. Googling reveals that it's been in beta and they seem to be expecting to roll it out soon.
The "etc." simply means that we're looking to expand the number of database options over time beyond this list
Rather than saying "etc." which makes it look like there are additional databases you currently support but are not listed, why not change it to "and more on the way" which clearly communicates what you mean?