Improve docs for EXPLAIN (IO)

Improve docs for EXPLAIN (IO)

Commit 681daed931 introduced a new EXPLAIN option "IO", but the docs
did not explain what information was added to the output. Expand the
description a little bit, similarly to the other EXPLAIN options.

While at it, fix a typo in the first sentence.

Author: Tomas Vondra <tomas@vondra.me>