Unclear in documentation where 'id' is

Summary: In docs page, an "id" argument is mentioned, but it's not clear where the id is or how to use it. This was brought up on the forums in regards to rereduce.

I'm not clear if 'id' is meant to refer to 'key', or if id comes from someplace else, or the 'key' in the example should be 'key'. I think that the function mentioned in the document refers to the __'reduce' argument in the actual map/reduce function, and therefore id should come from 'meta', but I'm not sure.

Notes: This was brought up in a forum post.