doc: io-properties-file.md: remove single mount point limitation
doc: io-properties-file.md: remove single mount point limitation

Since b2ef578f io_queue: create more than one I/O queue when we have more than one mountpoint
we do support multiple mountpoint entries in io-properties-file.

Also, clarify wording regarding the read/write iops/bandwidth attributes
that are per-mountpoint.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

avikivity/seastar,scylladb/seastar,dreamsxin/seastar,cloudius-systems/seastar,avikivity/seastar,dreamsxin/seastar,cloudius-systems/seastar,syuu1228/seastar,syuu1228/seastar,syuu1228/seastar,cloudius-systems/seastar,scylladb/seastar,dreamsxin/seastar,scylladb/seastar,avikivity/seastar