doc: correct QuicStream direction and id types

[kisnner26]

Fixes the documented nullability of QuicStream.direction and clarifies that QuicStream.id is only null while a stream is pending.

Fixes: #66686.

Validated against the current getters and test/parallel/test-quic-session-stream-lifecycle.mjs.

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[MikeMcC399]

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Edit: I made the change, since this PR has now been approved.

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