docs/fuse-io: Document the usage of DIRECT_IO_ALLOW_MMAP

By default, shared mmap is disabled in FUSE DIRECT_IO mode. However,
when the DIRECT_IO_ALLOW_MMAP flag is enabled in the FUSE_INIT reply,
shared mmap is allowed.

Signed-off-by: Tyler Fanelli <tfanelli@redhat.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

docs: filesystems: convert fuse-io.txt to ReST

- Add a SPDX header;
- Add a document title;
- Add it to filesystems/index.rst.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/88ec8025c1c5fc3ac5b65f1151c41ebcc696dc0e.1588021877.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>