Document better the lookup middleware return values

[bbatsov]

A follow-up to a comment made by @plexus.

Basically, we need to document precisely:

• all possible return keys
• what each key signifies

As a reminder, this is still an experimental op and it's subject to changes. Same with completions.

[bbatsov]

For reference - here's the existing documentation https://nrepl.org/nrepl/0.8/usage/misc.html#symbol-lookup

Normally, this would be in the https://nrepl.org/nrepl/0.8/ops.html section, but it's a bit hard to have a lot of info there, as this is generated from the middleware descriptors.

[bbatsov]

There's more on the subject here #275 (comment)