rseq: Move algorithm comment to top

rseq: Move algorithm comment to top

Move the comment which documents the RSEQ algorithm to the top of the file,
so it does not create horrible diffs later when the actual implementation
is fed into the mincer.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Link: https://patch.msgid.link/20251027084306.149519580@linutronix.de