docs(api.md): Adding new example to page.$$eval

[ffeu]

IMHO it's unclear that the second parameter actually receives the array instead of being executed for each item. Adding this example to maybe make that clear.

[mathiasbynens]

IMHO it's unclear that the second parameter actually receives the array instead of being executed for each item.

Arguably, the length example already makes that clear -- or at least, that the callback is being executed on something with a length property.

Adding this example to maybe make that clear.

The new example makes it clear that it's in fact a real array and not a NodeList, so I'll accept this PR for that reason :) Thanks!