Docs: `user/lookup` and `fields`

1. The name of the `fields` param can be found only in the examples, not cool. It should be mentioned explicitly preferably somewhere around the line

> To avoid getting more data than you need, you can (should!) specify a comma-separated list of fields you desire.

which says that you should specify a list, but doesn’t say _how_ to do it.
2. Looks like `fields=none` is also accepted, and that’s very cool when all you want to get is uid. This should be documented!

Start with the documentation for the `user/lookup` endpoint and its existing examples, then read the guidance around specifying a comma-separated list of fields. Done means the `fields` parameter is named explicitly and the accepted `fields=none` behavior is documented.

Written by the indexing model from the issue text.