Document location field in config.js

config.js

@@ -13,6 +13,12 @@
  *               window.TRANSLATIONS in i18n.js. English ('en') is the base;
  *               if a key is missing in the current language, English is used.
  *
+ * location    - Geographic coordinates for sun/moon calculations. Used
+ *               by the SunCalc library (SunCalc.getTimes, getMoonTimes,
+ *               getPosition). Object with 'lat' (latitude, -90 to +90)
+ *               and 'lon' (longitude, -180 to +180). Positive lat = north,
+ *               positive lon = east.
+ *
  * timeFormat  - Format string for the digital clock, using strict
  *               ISO 8601 tokens (case-sensitive). Any other character is
  *               treated as a literal (including whitespace).