Merge #243

243: future docs r=stjepang a=yoshuawuyts

Docs for futures concurrency. We currently don't have anything, and I figured this would be helpful in pointing folks in the right direction. Thanks!

## Screenshot

![Screenshot_2019-09-27 async_std future - Rust](https://user-images.githubusercontent.com/2467194/65730755-3c836780-e0c3-11e9-9bc5-fbf400cec7b2.png)


Co-authored-by: Yoshua Wuyts <[email protected]>

Author: bors[bot]

src/future/mod.rs

@@ -1,4 +1,45 @@
 //! Asynchronous values.
+//!
+//! ## Base Futures Concurrency
+//!
+//! Often it's desireable to await multiple futures as if it was a single
+//! future. The `join` family of operations converts multiple futures into a
+//! single future that returns all of their outputs. The `select` family of
+//! operations converts multiple future into a single future that returns the
+//! first output.
+//!
+//! For operating on futures the following macros can be used:
+//!
+//! | Name             | Return signature | When does it return?     |
+//! | ---              | ---              | ---                      |
+//! | `future::join`   | `(T1, T2)`       | Wait for all to complete
+//! | `future::select` | `T`              | Return on first value
+//!
+//! ## Fallible Futures Concurrency
+//!
+//! For operating on futures that return `Result` additional `try_` variants of
+//! the macros mentioned before can be used. These macros are aware of `Result`,
+//! and will behave slightly differently from their base variants.
+//!
+//! In the case of `try_join`, if any of the futures returns `Err` all
+//! futures are dropped and an error is returned. This is referred to as
+//! "short-circuiting".
+//!
+//! In the case of `try_select`, instead of returning the first future that
+//! completes it returns the first future that _successfully_ completes. This
+//! means `try_select` will keep going until any one of the futures returns
+//! `Ok`, or _all_ futures have returned `Err`.
+//!
+//! However sometimes it can be useful to use the base variants of the macros
+//! even on futures that return `Result`. Here is an overview of operations that
+//! work on `Result`, and their respective semantics:
+//!
+//! | Name                 | Return signature               | When does it return? |
+//! | ---                  | ---                            | ---                  |
+//! | `future::join`       | `(Result<T, E>, Result<T, E>)` | Wait for all to complete
+//! | `future::try_join`   | `Result<(T1, T2), E>`          | Return on first `Err`, wait for all to complete
+//! | `future::select`     | `Result<T, E>`                 | Return on first value
+//! | `future::try_select` | `Result<T, E>`                 | Return on first `Ok`, reject on last Err
 
 #[doc(inline)]
 pub use std::future::Future;