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

[keikoro]

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.