Docs: clarify difference between "DEFAULT_LANGUAGES" and "languages" settings

It would be helpful if the section on Default Languages in the docs made it clearer that DEFAULT_LANGUAGES is an additional setting and should not be confused with the main languages setting when parsing dates.

I don't entirely understand what it's meant to be used for anyway; more detailed explanation i.e. better documentation may help reduce the likelihood of them getting confused.

User (experience) story: I'm a new user to the library who perused the "Settings" section after skimming the introduction due to the particular problem I'm trying to solve. I ended up confusing the DEFAULT_LANGUAGES setting with languages – the two basically melded in my brain, likely helped by the fact they use identical formatting – and I consequently spent/wasted time debugging a problem (language-specific date formatting being outright ignored) that actually wasn't one.

Start with the Default Languages section in settings.html and compare it with the languages setting described in the linked introduction section. Clarify that DEFAULT_LANGUAGES is an additional setting, explain its intended use, and distinguish its effect from languages so a reader can tell which setting controls parsing.