Rollup merge of #89670 - yoshuawuyts:available-parallelism-docs, r=joshtriplett

Improve `std:🧵:available_parallelism` docs

_Tracking issue: https://github.com/rust-lang/rust/issues/74479_

This PR reworks the documentation of `std:🧵:available_parallelism`, as requested [here](https://github.com/rust-lang/rust/pull/89324#issuecomment-934343254).

## Changes

The following changes are made:

- We've removed prior mentions of "hardware threads" and instead centers the docs around "parallelism" as a resource available to a program.
- We now provide examples of when `available_parallelism` may return numbers that differ from the number of CPU cores in the host machine.
- We now mention that the amount of available parallelism may change over time.
- We make note of which platform components we don't take into account which more advanced users may want to take note of.
- The example has been updated, which should be a bit easier to use.
- We've added a docs alias to `num-cpus` which provides similar functionality to `available_parallelism`, and is one of the most popular crates on crates.io.

---

Thanks!

r? `@BurntSushi`

library/std/src/thread/mod.rs

@@ -1428,39 +1428,76 @@ fn _assert_both<T: Send + Sync>() {}
     _assert_both::<Thread>();
 }
 
-/// Returns the number of hardware threads available to the program.
+/// Returns an estimate of the default amount of parallelism a program should use.
 ///
-/// This value should be considered only a hint.
+/// Parallelism is a resource. A given machine provides a certain capacity for
+/// parallelism, i.e., a bound on the number of computations it can perform
+/// simultaneously. This number often corresponds to the amount of CPUs or
+/// computer has, but it may diverge in various cases.
 ///
-/// # Platform-specific behavior
+/// Host environments such as VMs or container orchestrators may want to
+/// restrict the amount of parallelism made available to programs in them. This
+/// is often done to limit the potential impact of (unintentionally)
+/// resource-intensive programs on other programs running on the same machine.
 ///
-/// If interpreted as the number of actual hardware threads, it may undercount on
-/// Windows systems with more than 64 hardware threads. If interpreted as the
-/// available concurrency for that process, it may overcount on Windows systems
-/// when limited by a process wide affinity mask or job object limitations, and
-/// it may overcount on Linux systems when limited by a process wide affinity
-/// mask or affected by cgroups limits.
+/// # Limitations
+///
+/// The purpose of this API is to provide an easy and portable way to query
+/// the default amount of parallelism the program should use. Among other things it
+/// does not expose information on NUMA regions, does not account for
+/// differences in (co)processor capabilities, and will not modify the program's
+/// global state in order to more accurately query the amount of available
+/// parallelism.
+///
+/// The value returned by this function should be considered a simplified
+/// approximation of the actual amount of parallelism available at any given
+/// time. To get a more detailed or precise overview of the amount of
+/// parallelism available to the program, you may wish to use
+/// platform-specific APIs as well. The following platform limitations currently
+/// apply to `available_parallelism`:
+///
+/// On Windows:
+/// - It may undercount the amount of parallelism available on systems with more
+///   than 64 logical CPUs. However, programs typically need specific support to
+///   take advantage of more than 64 logical CPUs, and in the absence of such
+///   support, the number returned by this function accurately reflects the
+///   number of logical CPUs the program can use by default.
+/// - It may overcount the amount of parallelism available on systems limited by
+///   process-wide affinity masks, or job object limitations.
+///
+/// On Linux:
+/// - It may overcount the amount of parallelism available when limited by a
+///   process-wide affinity mask, or when affected by cgroup limits.
+///
+/// On all targets:
+/// - It may overcount the amount of parallelism available when running in a VM
+/// with CPU usage limits (e.g. an overcommitted host).
 ///
 /// # Errors
 ///
-/// This function will return an error in the following situations, but is not
-/// limited to just these cases:
+/// This function will, but is not limited to, return errors in the following
+/// cases:
 ///
-/// - If the number of hardware threads is not known for the target platform.
-/// - The process lacks permissions to view the number of hardware threads
-///   available.
+/// - If the amount of parallelism is not known for the target platform.
+/// - If the program lacks permission to query the amount of parallelism made
+///   available to it.
 ///
 /// # Examples
 ///
 /// ```
 /// # #![allow(dead_code)]
 /// #![feature(available_parallelism)]
-/// use std::thread;
+/// use std::{io, thread};
 ///
-/// let count = thread::available_parallelism().map(|n| n.get()).unwrap_or(1);
+/// fn main() -> io::Result<()> {
+///     let count = thread::available_parallelism()?.get();
+///     assert!(count >= 1_usize);
+///     Ok(())
+/// }
 /// ```
+#[doc(alias = "available_concurrency")] // Alias for a previous name we gave this API on unstable.
 #[doc(alias = "hardware_concurrency")] // Alias for C++ `std::thread::hardware_concurrency`.
-#[doc(alias = "available_concurrency")] // Alias for a name we gave this API on unstable.
+#[doc(alias = "num_cpus")] // Alias for a popular ecosystem crate which provides similar functionality.
 #[unstable(feature = "available_parallelism", issue = "74479")]
 pub fn available_parallelism() -> io::Result<NonZeroUsize> {
     imp::available_parallelism()