Docs for task::spawn must mention support for blocking execution

[jimblandy]

The async-std blog post
"stop worrying about async blocking"
explains that spawn_blocking is no longer needed, and that developers should simply use spawn instead and let the library decide whether it should keep the thread in the async pool.

If task::spawn is the recommended way to spawn tasks that may block, then users will write code that depends on that, and the design decision becomes something async-std can't revert without introducing bugs in its users. This is all fine (I support the decision), but non-obvious characteristics covered by async-std's stability guarantees should be documented.

[jimblandy]

Sorry - this is a dup of #905. GitHub gave me a strange error when I filed this, so I just blundered onwards...