Documenation: switching-sched: Remove notes about elevator argument

This argument was ignored since blk-mq was set as default, so remove it
from documentation.

Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Marcos Paulo de Souza <marcos.souza.org@gmail.com>

.txt file is now .rst

Signed-off-by: Jens Axboe <axboe@kernel.dk>

docs: block: convert to ReST

Rename the block documentation files to ReST, add an
index for them and adjust in order to produce a nice html
output via the Sphinx build system.

At its new index.rst, let's add a :orphan: while this is not linked to
the main index.rst file, in order to avoid build warnings.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>