Documented API calls' return format.

This is in response to issue #108.

:ghost: :jack_o_lantern: :ghost: 
Orthogonal to this patch, but while you're at it: change this to "any project"; the wording in this and `list_project_nodes` is a bit ambiguous about whether it's one project or all of them that we're talking about (and the right meaning is different for the other call).