update comment in doc to match actual behavior

This is running under node v0.10.17 so I'm not sure if this is a change in node behavior or just a typo.

The existing listed behavior seemed intended to show that a 2-entry array of strings would be joined with commas, but as written was a 1-entry array with 'a,b' as the string.

I'm not **sure** that's the issue, though, since the comment shows what appears to be the strings joined, but with single-quotes around each individual string first, then double-quotes around the entire joined string.