[docs] Clarify Query.use vs Query.usePreloaded/Query.useLoader

[zth]

Relay allows you to easily start loading data as soon as possible, and not have to wait until a component actually renders (like with Query.use). There's a bunch of primitives for this in Relay/ReasonRelay, and they need to be documented properly; when to use what, and what the trade-offs are.

[zth]

Clarified somewhat in the docstring from the PPX. Should be clarified more in the docs, but we'll do that later.