Improve CS require comment (atom#21062)

Turns out the comment was misleading; the build also fails to find modules like `donna` without the import.
I've instead changed the message to make it more vague, so removal requires more consideration.

script/build

@@ -10,7 +10,7 @@ if (process.argv.includes('--no-bootstrap')) {
   require('./bootstrap')
 }
 
-// Needed so we can require src/module-cache.coffee during generateModuleCache
+// Required to load CS files in this build script, such as those in `donna`
 require('coffee-script/register')
 require('colors')