docs: add note about how how font-family declarations are handled

resolves #172

Author: danielroe

docs/content/1.get-started/3.usage.md

@@ -12,6 +12,10 @@ div {
 }
 ```
 
+::callout
+Note that Nuxt Fonts will only generate CSS for the first font in a `font-family` declaration. Subsequent fonts in the list will be used to generate metrics for font fallbacks.
+::
+
 ## Tailwind CSS
 
 ### v3 and below