doc: seq_file: clarify role of *pos in ->next()

There are behavioural requirements on the seq_file next() function in
terms of how it updates *pos at end-of-file, and these are now enforced
by a warning.

I was recently attempting to justify the reason this was needed, and
couldn't remember the details, and didn't find them in the
documentation.

So I re-read the code until I understood it again, and updated the
documentation to match.

I also enhanced the text about SEQ_START_TOKEN as it seemed potentially
misleading.

Cc: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Link: https://lore.kernel.org/r/87eemqiazh.fsf@notabene.neil.brown.name
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

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