Add small clarification to Ecto.Query docs

[Sleepful]

This makes sense to me, not sure if this makes sense to other people. LMK if this makes sense.

Motivation

I was trying to find out if Ecto.Query functions would query the DB. Some examples call Repo functions so they imply that Repo must be used but it isn't too explicit. For instance Ecto.Query.preload/3 has this description:

Preloads the associations into the result set.

which sounds a lot like Ecto.Repo.preload/3

Preloads the associations into the result set.

Ecto.Repo.preload/3 also includes this bit, which makes sense but not while doubting whether Query.preload will fetch or not.

This is similar to Ecto.Query.preload/3 except it allows you to preload structs after they have been fetched from the database.

[josevalim]

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