Document use of resources for JS

[vados-cosmonic]

We should document resource usage in Javascript, or at least point to a place where the documentation can be found.

At this point the information shouldn't change too much, but it might be a bit too much to add to the simple language guide. I can think of at least three places where this information might go:

• Advanced topics section (in a Javascript subtopic)
• FAQ section specific to JS (links out to jco documentation that is canonical)
• In the language guide that exists now

[kate-goldenring]

This would be great. I would add it to the language guide.

[vados-cosmonic]

There is now a section in the Jco book for this, so we can link to there from the language guide and wrap up this issue.