Docs: clarify behavior of Lwt.pick []
Docs: clarify behavior of Lwt.pick []

Prompted by #551, #552.

[skip ci]

ocsigen/lwt,rneswold/lwt,c-cube/lwt,c-cube/lwt,ocsigen/lwt,rneswold/lwt,rneswold/lwt,c-cube/lwt,ocsigen/lwt