unseal std::io::IsTerminal

[GrigorenkoPV]

• Closes Feature request: io::IsTerminal should not be sealed #161989
• See also: std::io: impl IsTerminal for {Empty, Repeat, Sink, Take} #161947

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[clarfonthey]

@rfcbot fcp merge libs

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[joshtriplett]

We should update the documentation to spell out the expectations for implementing it.

In particular, we discussed in the @rust-lang/libs meeting that someone is free to implement IsTerminal for a type that also implements AsFd, and .is_terminal() does not have to return the same as .as_fd().is_terminal(). We should document that those generally should match, but that if you want to know whether you can use (for instance) TTY ioctls on the underlying file descriptor, you should check the underlying file descriptor.