Clarify toFormat docs

[andrey-p]

I'm a dev migrating from Moment.js who just lost a couple of hours to the fact that toFormat doesn't default to the user's own locale. I figured I might help others by adding some tweaks to the docs.

My changes are two-fold:

1. Clarify that format in Moment.js and toFormat in Luxon aren't fully equivalent to each other. In Moment.js, format is the preferred way of formatting dates, even for human-readable strings.
2. Add a note to the Intl section of toFormat to clarify that the default locale is en-US.

Re #2, in the contributing guidelines you say that you prefer gotchas to go in the docstrings, but considering toFormat already has a dedicated section for internationalization and I'm not the first person to run into this, I hope you'll consider making a minor exception.

[icambron]

agreed, thanks