Clarify toFormat docs (#938)

Author: andrey-p

docs/formatting.md

@@ -141,6 +141,8 @@ DateTime.fromISO('2014-08-06T13:07:04.054')
   .toFormat('yyyy LLL dd'); //=> '2014 août 06'
 ```
 
+Note `toFormat` defaults to `en-US`. If you need the string to be internationalized, you need to set the locale explicitly like in the example above (or more preferably, use `toLocaleString`).
+
 ### Escaping
 
 You may escape strings using single quotes:

docs/moment.md

@@ -139,7 +139,7 @@ See the [formatting guide](formatting.html) for more about the string-outputting
 | full ISO 8601    | `iso()`        | `toISO()`                   |                                                                                                                                                   |
 | ISO date only    | None           | `toISODate()`               |                                                                                                                                                   |
 | ISO time only    | None           | `toISOTime()`               |                                                                                                                                                   |
-| custom format    | `format(...)`  | `toFormat(...)`             | The format tokens differ between Moment and Luxon, such that the same format string cannot be used between the two.                               |
+| custom format    | `format(...)`  | `toFormat(...)`             | The format tokens differ between Moment and Luxon, such that the same format string cannot be used between the two. Note `toFormat` is meant to be used for machine-readable formats. For anything human-readable, you really want to use `toLocaleString()` instead. |
 | RFC 2822         |                | `toRFC2822()`               |                                                                                                                                                   |
 | HTTP date string |                | `toHTTP()`                  |                                                                                                                                                   |
 | JS Date          | `toDate()`     | `toJSDate()`                |                                                                                                                                                   |