Document re-exports better

We use `#[doc(inline)]` for std and futures re-exports; this is nice because it creates a more cohesive feel for our library (which makes it easier to discover and learn). But it can sometimes be confusing for people what we re-export, and what not.

One way we could do this is by writing docs for all our re-exports, and what we've newly defined in each mod-level docs. This might be useful to create a distinction:

- re-exports: what did we re-export from `std` or `futures`?
- ports: what did we 1:1 port from std, and just make async?
- new: what did we newly define that has no counterpart in std?

Additionally we could mark direct re-exports such as `task::Context` with a label in our documentation to show that they've been re-exported:

```rust
#[cfg_attr(feature = "docs", doc(cfg(re-export)))]
```

Though I'm unsure how good this is, as it would take the same place as `"unstable"` and could be confusing to people. But still, it's probably something worth considering.

Thanks!

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