Be explicit in docs about locales that a single locale is supported

[ericf]

In all of the libs (low-level and integration) we use the plural locales, and this confuses people since their mental model is that the page is in a single locale. The reason we do this is to align with the Intl* APIs which support fallback locales.

To help make setting up the locale(s) easier for people to understand we should be explicit about the fact that a single locale is support and the most common thing people will be using.

Related: formatjs/formatjs#39

[martindederer]

Any news on this? I am still trying to wrap my head around the locales (plural) thing as described in formatjs/formatjs#39. What is the use case for supplying several locales in the "locales" property of a react component?