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>