io: AsyncFd minor tweaks

[folkertdev]

Motivation

Fixing or documenting some things I found confusing

Solution

• simplify the logic of try_io by only checking once whether the error is WouldBlock
• document why AsyncFd uses an Option around its inner field.
• remove some .into() calls (clearer this way, I think, but also reduced code bloat)

[Darksonn]

Thanks.