[docs] specify adapter is required when building

Closes #854

Author: ignatiusmb

documentation/docs/14-configuration.md

@@ -70,7 +70,7 @@ export default config;
 
 ### adapter
 
-Determines how the output of `svelte-kit build` is converted for different platforms. See [Adapters](#adapters).
+Required when running `svelte-kit build` and determines how the output is converted for different platforms. See [Adapters](#adapters).
 
 ### amp