Clarify the guarantees that ThreadId does and doesn't make.

The existing documentation does not spell out whether `ThreadId`s are unique
during the lifetime of a thread or of a process. I had to examine the source
code to realise (pleasingly!) that they're unique for the lifetime of a process.
That seems worth documenting clearly, as it's a strong guarantee.

Examining the way `ThreadId`s are created also made me realise that the `as_u64`
method on `ThreadId` could be a trap for the unwary on those platforms where the
platform's notion of a thread identifier is also a 64 bit integer (particularly
if they happen to use a similar identifier scheme to `ThreadId`). I therefore
think it's worth being even clearer that there's no relationship between the
two.

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