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

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

Improve `std::thread::available_parallelism` docs

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

This PR reworks the documentation of `std::thread::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.

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Thanks!

r? `@BurntSushi`