Note why `Handle` methods in `win` won't panic

[Ashley Mannix]

Relevant API Guidelines:

• Panic Docs
• Destructors never fail

The win implementation of Handle calls unwrap in the following methods:

• Drop::drop
• AsRawFd::as_raw_fd
• IntoRawFd::into_raw_fd
• as_file
• as_file_mut

These calls should either have a comment inline documenting why they can't fail, or a Panics section should be added to the docs documenting when they will fail.

[Vsevolod Zubarev]

I'll have a look into this.