Clarify behaviour of flush on WouldBlock error

[AljoschaMeyer]

Is an implementation of the AsyncWrite trait allowed to block on flushing?

The first sentence of the trait doc comment indicates that this should not be the case:

This trait inherits from io::Write and indicates that an I/O object is nonblocking, meaning that it will return an error instead of blocking when bytes cannot currently be written, but hasn't closed.

But the remainder of the documentation only explicitely talks about how write should handle WouldBlock errors, not about flush.

cc facebookarchive/rust-partial-io#17

[sunshowers]

As of the latest version of partial-io, we're now treating WouldBlock during flush the same as WouldBlock during write as far as notifying the current task goes.

[carllerche]

@Sid0 This is correct (and should be documented). It is expected that both write and flush will used from the same task.