Documentation/io-mapping: Remove outdated blurb

The implementation details in the documentation are outdated and not really
helpful. Remove them.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: https://lore.kernel.org/r/20201103095858.734064977@linutronix.de